STARRY NIGHT COMPANION

STARRY NIGHT COMPANION

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Starry Night CompanionYour Guide to Understanding the Night Sky Using Starry Night

Written by John Mosley

Edited by Mike Parkes

Foreword by Andrew L. Chaikin

[email protected]

284 Richmond St. E.Toronto, ONM5A 1P4, Canada(416) 410-0259

©1993-2000 SPACE.com Incorporated.All rights reserved.SPACE.com, the SPACE.com logo, Starry Night, the Starry Night logo,and LiveSky are trademarks of SPACE.com, Inc.

Cover photo composite © Dennis di Cicco and Terence Dickinson

ISBN 1-894395-04-2

Printed in Canada.

To Barbara, with love

CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Section 1 – Astronomy Basics1.1 The Night Sky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.2 Motions of the Earth . . . . . . . . . . . . . . . . . . . . . . . . 331.3 The Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Section 2 – Observational Advice2.1 SkyWatching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552.2 Co-ordinate Systems . . . . . . . . . . . . . . . . . . . . . . . . 63

Section 3 – Earth’s Celestial Cycles3.1 Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733.2 Revolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.3 Precession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Section 4 – Our Solar System4.1 The Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1154.2 Planets, Asteroids & Comets . . . . . . . . . . . . . . . . . 131

Section 5 – Deep Space5.1 Stars and Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . 1575.2 About the Constellations . . . . . . . . . . . . . . . . . . . . 1735.3 Highlights of the Constellations . . . . . . . . . . . . . . . 189

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211Appendix A – The Constellations . . . . . . . . . . . . . . . . . 213Appendix B – Properties of the Planets . . . . . . . . . . . . . 219Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235


FOREWORD

If the sky could speak, it might tell us, “Pay attention.” That isusually good advice, but it’s especially valuable when it comes to

the heavens. The sky reminds us that things are not always as they

seem.It wasn’t too long ago that we humans believed the Earth was

the center of it all, that when we saw the Sun and stars parade across

the sky, we were witnessing the universe revolving around us. Ittook the work of some bold and observant scientists, including the

Polish astronomer Nicolaus Copernicus, to show us how wrong we

had been: Instead of the center of the universe, Copernicus said,our world is one of many planets and moons circling the Sun in a

celestial clockwork.

Some 250 years later the Apollo 8 astronauts became the firsthumans to see the truth of this with their own eyes. As they made

history’s first voyage around the moon, they looked back at the

Starry Night Companion8

Earth, whose apparent size diminished until they could cover it with

an outstretched thumb. In that moment, they knew on a gut levelthat Copernicus was right. Most of us, though, have not had the

privilege of such a view. Standing under a canopy of stars on a clear

night, we’ve had to take on faith what astronomers have told us.Starry Night, whose many features are described in this book,

changes all that. John Mosley’s clearly written and engaging text is

filled with details about this remarkable desktop planetarium, whichallows you to make virtual journeys into the cosmos. Want to see

why the moon changes in appearance during a month? Starry Night

lets you “stand” on an imaginary vantage point in deep space andsee the moon’s motion around Earth, watching as the lunar nearside

goes from darkness into sunlight and back again.

Farther from home, you can watch Jupiter’s family of moonsrevolve like a solar system in miniature. You can ride on a comet as

it makes its decades-long trek around the Sun. You can even journey

back in time, to see the sky as it looked when Galileo turned hisfirst telescope on the heavens. Or, you can peek into the future, to

get an advance look at solar eclipses, lunar eclipses, and other

skyshows.A Starry Night Companion, then, offers a tour guide to the heav-

ens. In the these pages, you’ll do something Copernicus could only

dream of: You will learn your way around the universe. You’llcome to know it as a place filled with wonders, from red giant stars

to glowing nebulae. You’ll experience what it would be like to

view the sky from hundreds of light years away. And like the astro-nauts, you’ll return to Earth with a new perspective on our planet

and its place in the cosmos.

Best of all, you’ll enrich your experience of the wonders aboveour heads. The sky is there for us to enjoy, and that enjoyment is

9Foreword

only increased when we understand what we are seeing. When we

stand under a dome of stars, we can see it with the benefit of centuriesof detective work that have unraveled the heavens’ secrets. And we

can ponder the mysteries that remain for us to solve, if we only pay

attention.

Andrew Chaikin

Executive Editor, Space and Science

SPACE.com

August, 2000


INTRODUCTION

The sky is enormous, distant, and filled with mysterious things.It doesn’t resemble anything we encounter in our daily lives. There

is movement and change in the sky, but generally at a rate too slow

for us to notice. To many, the sky is intimidating. It may seeminaccessible. But it is superbly fascinating. You bought this book

and software package because you would like to get to know it

better. Welcome – the adventure is about to begin.Two hard facts about the sky are that (1) changes in it happen

slowly, and (2) we cannot cause it to change. We cannot turn it and

look at it from a different angle; we cannot speed up the rotation ofthe earth or the motion of the planets through the constellations; we

cannot cause an eclipse to happen; we cannot view the sky from

another place on earth (unless we actually go there); we cannot seewhat the sky looked like long ago or in the distant future. We are

stuck in the here and now.

That is the value of this book and software package. StarryNight lets you manipulate the sky. You can move backward and


forward through time as far and as fast as you wish; you can move

to any spot on the earth and to any planet; you can see imaginarylines in the sky (such as the paths of the planets and constellation

boundaries); you can discover future events and watch them before

they actually happen, or replay famous events from the past. Youcan take control of the sky. As you go, you will gain a far better

understanding of how the sky works than you can as a passive

observer.Then, when you actually step outside on a clear night, you will

have a greater understanding of what you see. It will be fun. Starry

Night lets you stargaze on cloudy nights and even during the day-time, but there is no substitute for stepping outside at night and

seeing the real thing. Starry Night will help you appreciate what

you see when you are outdoors on a starry night, but don’t forget toactually go outside and look up!

How to Use This Book

Starry Night Companion is organized into five sections. Starry

Night example files will be used in almost every section tocomplement the material in the text. These files can be opened by

clicking the “Go” menu in Starry Night and then choosing

“Companion Book”. The files are subdivided by chapter. Just clickon the file you are interested in to launch this file. If there is no

“Companion Book” item in your “Go” menu, you are using an older

copy of Starry Night, and should update by visitingwww.starrynight.com

The first section covers the essentials of understanding the night

sky. Several step-by-step exercises let you get familiar with StarryNight and show how it can help you learn about astronomy.

Newcomers to the hobby may want to read section one and then

13Introduction

spend some time using Starry Night and making their own

observations, before returning to the rest of this book.The next four sections go into more detail.

Section two will help you get the most out of your observing,

no matter what you are looking at. It has observational tips, hintson choosing binoculars and telescopes, and a good description of

the various astronomical co-ordinate systems.

Section three covers the three different motions of the earthwhich account for much of the variation we see in the sky.

Section four looks at other objects in our solar system, including

our moon and the nine planets.Finally, the fifth section takes you into the deep, describing

stars, galaxies, and more.


Section 1

Astronomy Basics

THIS SECTION of

Starry Night

Companion will give you a

quick overview of the

basics of astronomy.

Chapter 1.1 has general

information, while chapter

1.2 looks at the motion of

the earth and chapter 1.3

introduces the other bodies

in our solar system.

1.1

THE NIGHT SKY

The Constellations

Stepping outdoors at night in a dark place far from the bright

lights of the city, we are amazed at the number of stars we can see.

Bright stars and dimmer ones, tightly knit clusters with many pointsof light and dark patches with nothing at all. Stars, stars everywhere,

stretching off to the horizon in all directions, as far as the eye can

see. But which one of those bright points is the North Star? Is thatreddish dot Mars or a relatively cool star? And isn’t the International

Space Station up there somewhere tonight? Wouldn’t it be nice to

have a map?In fact, the sky has been mapped. Astronomers divide the sky

into 88 different non-overlapping areas called constellations. You

can think of the constellations as the “countries” on the surface ofthe sky. Each constellation is an area of the sky. Just like countries

Section 1 – Astronomy Basics18

on Earth, some constellations are bigger than others. Everythinginside the boundaries of a constellation is considered to be part of

that constellation, regardless of its distance from Earth. A

constellation is actually a wedge of the universe with Earth at itscenter that extends to the farthest reaches of space.

The brighter stars in each constellation make a unique pattern,

and we look for this pattern to identify the constellation. To helpmake this identification easier, people through the ages have

“connected the dots” of stars to draw recognizable patterns called

constellations, which they then named. Sometimes the patternactually looks like the object after which it is named; more often it

does not. One point of

confusion amongnewcomers is that the

boundaries and the

connect-the-dot stickfigures share the same

constellation name. If

you read that the sunis “in” Scorpius, the

The constellation Crux has the appearanceThe constellation Crux has the appearanceThe constellation Crux has the appearanceThe constellation Crux has the appearanceThe constellation Crux has the appearanceof a cross, but Pisces bears littleof a cross, but Pisces bears littleof a cross, but Pisces bears littleof a cross, but Pisces bears littleof a cross, but Pisces bears littleresemblance to a fish.resemblance to a fish.resemblance to a fish.resemblance to a fish.resemblance to a fish.

Constellation maps of the northern (left) and southern celestialConstellation maps of the northern (left) and southern celestialConstellation maps of the northern (left) and southern celestialConstellation maps of the northern (left) and southern celestialConstellation maps of the northern (left) and southern celestialhemispheres.hemispheres.hemispheres.hemispheres.hemispheres.

19The Night Sky

scorpion, it means that the sun is

inside the official boundaries of theconstellation Scorpius, but it is not

necessarily inside the stick figure that

outlines a scorpion. Starry Night canshow you the constellation

boundaries, stick figures, or both.

The constellations are thefundamental units of the visual sky,

yet they are imaginary. They were

invented. People created them longago – in some cases in prehistoric

times – and often for reasons that we

might think strange today.One constellation leads to

another, so to speak. Use easily-

recognizable star patterns to find moreobscure ones. The two stars at the end

of the bowl of the Big Dipper, for

example, point to the North Star andto the Little Dipper, while the five

stars of the handle of the Big Dipper

point to the star Arcturus in Bootes.The three belt stars of Orion point

west to Aldebaran in Taurus and east

to Sirius in Canis Major. It is easy todraw lines and arcs from star to star

and “star hop” from constellations you

know to those you are still learning.As in so many endeavors, don’t

bite off too much. Don’t try to learn

the entire sky on your first night out.

Mars, Jupiter, and SaturnMars, Jupiter, and SaturnMars, Jupiter, and SaturnMars, Jupiter, and SaturnMars, Jupiter, and Saturnare all considered to be inare all considered to be inare all considered to be inare all considered to be inare all considered to be inAries, though they areAries, though they areAries, though they areAries, though they areAries, though they areoutside its stick figure.outside its stick figure.outside its stick figure.outside its stick figure.outside its stick figure.

The belt of Orion is sand-The belt of Orion is sand-The belt of Orion is sand-The belt of Orion is sand-The belt of Orion is sand-wiched between the brightwiched between the brightwiched between the brightwiched between the brightwiched between the brightstars Sirius and Aldebaran.stars Sirius and Aldebaran.stars Sirius and Aldebaran.stars Sirius and Aldebaran.stars Sirius and Aldebaran.

The Big Dipper’s MerakThe Big Dipper’s MerakThe Big Dipper’s MerakThe Big Dipper’s MerakThe Big Dipper’s Merakand Dubhe make a straightand Dubhe make a straightand Dubhe make a straightand Dubhe make a straightand Dubhe make a straightline leading to Polaris.line leading to Polaris.line leading to Polaris.line leading to Polaris.line leading to Polaris.


Begin by learning the major constellations and then fill in the small

and obscure ones as the need and challenge arises.A curious feature of the constellations is that once you know

them, they are hard to forget. Like riding a bicycle, once you know

them you tend not to forget them, and they will remain with you fora lifetime. If you know the constellations, the sky is never an un-

familiar place and you will be at home under it wherever you go.

The constellations are described in much more detail in section 5 ofthis book.

Co-ordinate Systems

Although the constellation boundaries are a good starting point

for describing the approximate location of an object in the sky,astronomers need a more precise method of specifying an object’s

position. There are several different co-ordinate systems in common

use, and the most intuitive is the horizon co-ordinate system, alsoknown as the Alt-Az system

“Alt” is short for altitude, which is an object’s height above

the horizon in degrees (0 to 90). An object with an altitude of 0° isright along the horizon, while one with an altitude of 90° is directly

overhead in the sky. The imaginary point in the sky directly overhead

is known as the zenith. Knowing the altitudes of objects at differenttimes will help you plan your observing sessions. The best time to

observe something is when it has an altitude between 20° and 60°.

This is because objects (especiallyplanets) near the horizon can be

blurred by intervening air currents,

while objects with an altitude greaterthan 60° are tough to look at without

straining your neck! Avoid observing planetsAvoid observing planetsAvoid observing planetsAvoid observing planetsAvoid observing planetsnear the horizon.near the horizon.near the horizon.near the horizon.near the horizon.

21The Night Sky

“Az” stands for azimuth, an Arabic word (as is altitude) for the

position along the horizon; “bearing” is an alternative navigationalterm. Azimuth is measured in degrees from north, which has an

azimuth of 0°, through east (which has an azimuth or a bearing of

90°), through south (180°) and west (270°). The local meridian isthe imaginary line which divides the sky into an eastern and a western

half. It extends from the southern point on the horizon through the

zenith to the northern point on the horizon. The half of the meridianline which is south of the zenith has an azimuth of 180°, while the

half north of the zenith has an azimuth of 0°.

Once you have the altitude and azimuth of an object, you knowwhere to find it in the sky. A compass is helpful in determining the

azimuth of your viewing direction (compasses point to “magnetic

north”, which is not exactly the same as “true north”, but the twodirections are practically identical unless you are observing from a

far northern latitude). Once you have your bearing, it is just a matter

of looking up to the proper altitude. With practice, you will quicklylearn how high above the horizon is 10°, how high is 30°, and so

on. Starry Night can show you the altitude and azimuth of any

object at any time (just double-click on the object to bring up itsInfo Window with this information) and can also mark the zenith

and nadir points and/or the meridian line (choose the appropriate

option from the “Guides” menu).Although the horizon co-ordinate system is the easiest to

understand, it is not necessarily the most useful system. This is

because it is a “local” system, and the co-ordinates depend on yourpersonal location. Jupiter, for example, will have one set of horizon

co-ordinates for an observer in California, a second set of co-

ordinates for an observer in Texas, and a third for an observer some-where else. To make things worse, the co-ordinates change

constantly over time as the sky rotates. For these reasons, stars and

other objects in the sky are more commonly identified using the


equatorial co-ordinate system. This system gives one unique set of

co-ordinates for an object, which is the same anywhere on Earth.The equatorial system is described in chapter 2.2.

Angles in the Sky

Astronomy is full of angular measures, and it is very handy to

have a conceptual feeling for what these angles mean. A circle isdivided into 360°, so the distance from the horizon to the zenith is

one quarter of a circle or 90°. It is also 90° along the horizon from

one compass point to the next (east to south, for example). Familiarconstellations will help you visualize smaller angles. The distance

from the end star in the handle of the Big Dipper to the end star in

the bowl is 25°. The distance between the two end stars in the bowlis 5°. The sides of the Great Square of Pegasus average 15° in length.

The distance from one end of the W of Cassiopeia to the other is

13∞. The distance from Betelgeuse to Rigel in Orion is 19°, and thelength of Orion’s belt is just under 3°.

Your hand is a portable angle-measurer. The width of your

clenched fist held at arm’s length is about 10° (people with shorterarms generally have smaller fists and the general rule holds). The

width of a finger at arm’s length is about 2°. The angular diameter

of the moon is only 1/2°, although most people would guess it ismuch larger. Try it yourself. Block out the full moon with your little

finger held at arm’s length.

The field of view is an astronomical term which often confusesnewcomers to astronomy. It is the angular width of the patch of sky

you can see through your optical instrument, whether that instrument

is the naked eye, a pair of binoculars or a telescope. Field of view isusually expressed in degrees for binoculars and arcminutes for

telescopes. An arcminute is 1/60 of a degree. An even smaller unit,

an arcsecond, is 1/60 of an arcminute.

23The Night Sky

The entire sky from horizon to horizon encompasses 180°, but

we see only part of it at once, and with binoculars and telescopeswe see very little at a time. With the naked eye, we have a field of

view of about 100°, which is the field of view you see when you

open Starry Night. Binoculars typically give fields of view of 5° to7°, which is a large enough portion of the sky to see the bowl of the

Big Dipper or the belt of Orion, but not an entire constellation. A

telescope with a low-power eyepiece typically gives a field of viewof 30 arcminutes (expressed as 30’), which is the equivalent of 1/

2°. A high-power eyepiece on the same telescope may give a field

of view of 10 arcminutes (or 1/6°). It is difficult to find objects witha high magnification telescope because you can see only a very

small part of the sky at one time. Starry Night displays your field of

view and can also display circular outlines which represent the fieldsof view of different optical instruments.

The relative views of Orion’s belt as seen through 7 X 50 binocularsThe relative views of Orion’s belt as seen through 7 X 50 binocularsThe relative views of Orion’s belt as seen through 7 X 50 binocularsThe relative views of Orion’s belt as seen through 7 X 50 binocularsThe relative views of Orion’s belt as seen through 7 X 50 binoculars(7(7(7(7(7° field of view) and a low-power telescope eyepice (30' field of field of view) and a low-power telescope eyepice (30' field of field of view) and a low-power telescope eyepice (30' field of field of view) and a low-power telescope eyepice (30' field of field of view) and a low-power telescope eyepice (30' field ofview).view).view).view).view).

Stars

Gaze upwards on a clear night and you see – stars! A lot of

stars, if you are lucky enough to be in a dark place. How many is a

lot? From a dark location a person with good eyesight can seeabout 2,000 stars at any moment, or about 6,000 if he could see the

entire sky.


It is useful to have a system for classifying the brightness of

stars, and the one we use has been around for thousands of years.When compiling his star catalog in about 150 BC, the astronomer

Hipparchus devised the scheme still used today. He divided the

naked-eye stars into six magnitude groups, with lower numbersdesignating brighter stars. First magnitude stars are the brightest

and sixth the dimmest that most people can see without a telescope.

It is counter-intuitive for the brighter stars to have a smallermagnitude number, but the scheme has been used for so long that

astronomers have gotten used to it.

Modern astronomers have recalibrated Hipparchus’ scheme toput it on a sound mathematical footing, and magnitudes are now

expressed as decimals. The brightest star, Sirius, is now assigned a

magnitude of -1.4. The planets can be even brighter, Jupiter reaching-2.9 and Venus -4.5. The full moon is magnitude -12.6 and the sun

is -26.7. The magnitude of the dimmest object you can see is called

the limiting magnitude. With the naked eye on a clear night, thelimiting magnitude is about 6th magnitude, but with light pollution

it can be much higher. Binoculars and small telescopes will see

down to 9th magnitude, a large amateur telescope to 14th magnitude(the magnitude of Pluto), and the Hubble Space Telescope reaches

to 30th magnitude. Starry Night can show you the magnitude of any

object.

The stars of Perseus visible with a limiting magnitude of 6, and aThe stars of Perseus visible with a limiting magnitude of 6, and aThe stars of Perseus visible with a limiting magnitude of 6, and aThe stars of Perseus visible with a limiting magnitude of 6, and aThe stars of Perseus visible with a limiting magnitude of 6, and alimiting magnitude of 3.limiting magnitude of 3.limiting magnitude of 3.limiting magnitude of 3.limiting magnitude of 3.

25The Night Sky

Astronomical DistancesMost people know that distances in outer space are

quite large- “astronomical”, so to speak. Our normalmeasurements of length become meaningless in thesesituations, and astronomers have invented two new unitsof length which serve different purposes in astronomy.

The astronomical unitastronomical unitastronomical unitastronomical unitastronomical unit (AU) is the average distancefrom Earth to the sun. This equals 150 million kilometres(93 million miles). It is used primarily to describedistances to objects within the solar system. For example,Jupiter is about 5 AU from Earth on average, whileNeptune is about 30 AU away.

When talking about the distances to the stars, eventhe astronomical unit is not large enough. The neareststars are about 250 000 AU away, which is a remarkable37 500 000 000 000 km ( 24 000 000 000 000 miles)!For distances to stars and other deep space objects,astronomers use the light yearlight yearlight yearlight yearlight year. A beam of light (or radiowaves or any other sort of electromagnetic radiation)travels at “the speed of light,” which is 299,792 kilo-meters (186,282 miles) per second. That is fast enoughto travel seven times around Earth in one second or tothe moon and back in three seconds. At this incrediblerate, a beam of light travels 9,460,540,000,000 kilometers(5,878,507,000,000 miles) in one year – and that distanceis one light year. A light year is a unit of distancedistancedistancedistancedistance, nottimetimetimetimetime, and it is approximately 9-1/2 trillion kilometers (6trillion miles).


Each magnitude step represents a difference in brightness of

2-1/2 times; for example, a 3rd magnitude star is 2-1/2 times asbright as a 4th magnitude star. (The actual ratio is 2.512, which is

the 5th root of 100; the difference between a 1st and 6th magnitude

star is 100 times.) The Hubble Space Telescope can photographstars 24 magnitudes or 4 billion times fainter than the dimmest stars

your eye can see.

The magnitude of a star depends on two things: the amountof light it gives off and its distance from Earth. The stars which can

be seen from Earth range in distance from about 4 light years to

several thousand light years, making it impossible to determine howintrinsically bright a star is by its magnitude alone.

Learning to See

Although sixth magnitude is the theoretical naked-eye limit

for most people, you may have trouble seeing objects this faint atfirst. It takes practice to learn to sing a song, throw a baseball, or

look at a small, faint object through a telescope – or at least it takes

practice to do it well. You must learn how to see. Slow down, andlet your eye absorb the image. Astronomical objects are small and

contrast is low, so details do not spring out. The most important

thing to do is to relax, linger at the eyepiece, and let an image slowlyaccumulate on your eye.

The greatest amateur astronomer of all and the discoverer of

the planet Uranus, William Herschel, said, “You must not expect tosee ‘at sight.’ Seeing is in some respects an art which must be learne-

d. Many a night have I been practicing to see, and it would be strange

if one did not acquire a certain dexterity by such constant practice.”Go slow, and take the time to see.

27The Night Sky

The Night SkyStarry Night Exercise

This exercise is designed to complement the concepts discussed inthis chapter. The instructions are written for Starry Night Backyard, butwill also work with Starry Night Pro. If there are differences in instructionsbetween the two versions, this will be explained in the exercise.You shouldopen Starry Night on your computer before beginning the exercise.

As mentioned earlier, the sky is divided into different constellations.Starry Night lets you display the constellations in several different ways.

1) Open a new window by selecting “File | New”.2) Choose “Go | Viewing Location” to bring up the Viewing Location

window.3) Click the “Lookup” button and select Chicago from the list of cities.

Click the “OK” button. The co-ordinates for Chicago should nowappear in the Viewing Location window. Make sure that the boxmarked “DST” is not checked. Click the “Set Location” button tochange your viewing location to Chicago.

4) Change the date to Dec.1, 2000 and change the time to 10:00 PM.Press the “Stop” button in the time controls to keep time fixed at10:00 PM.

5) Select “Constellations | Boundaries” (“Guides | Constellations |Boundaries” on Pro. All constellation options in Pro require this extrastep) to draw the constellation boundaries on the screen and then select“Constellations | Labels” to label them. Select “Constellations |Constellation Settings” and press the “Labels” button in theConstellation Settings window. In the Label Settings window whichcomes up, make sure the option “Both” is selected (under the heading“Constellation Label Options”). This displays the common andastronomical names of the constellations. Press the “OK” button toclose these two windows. Scroll around the sky using the mouse andnotice the different sizes of the constellations and their irregular shapes.

6) Starry Night gives you several different ways to draw the stick figuresof the constellations. Select “Constellations | Astronomical” to turnon the common stick figures (on Starry Night Pro, choose“Constellations | Stick Figures”). You can see that all the stick figuresstay within their constellation boundaries. Again, scroll around the


sky and note that few figures bear any resemblance to their names,although Orion (in the southeast) does resemble a hunter with bow,and the Great Bear (in the northeast) is shaped like an animal.

7) Select “Constellations | Rey’s” (on Starry Night Pro, you need tochoose “Guides | Constellations | Constellation Settings” and clickthe button marked “Rey’s”) to display the constellation figurespopularized by H.A. Rey. He designed these figures so that the picturesthey made corresponded with the constellation names. If you viewthe real night sky, however, you may have trouble matching the stararrangements with Rey’s figures!

8) As an example of how you can use the constellations to look for anobject, suppose you want to observe the star cluster M52. Open theFind dialog by choosing “Edit | Find” (“Selection | Find” on StarryNight Pro) and type in “M52” in the field marked “Name Contains”.Make sure that the checkbox marked “Magnify for Best Viewing ofFound Object” is not checked. Click the “Find” button to have StarryNight center and mark the object. You can see that M52 is inCassiopeia. Select "Constellations | Labels" to turn off the labels, andnotice the distinctive “W” figure which Cassiopeia makes in the sky.This “W” is one of the easiest star patterns to find in the night sky.You can see that the rightmost line of the “W” (the line that connectsthe stars Schedar and Caph) points to the cluster. This will help youpoint your binoculars on M52 when you go observing (note that M52is too faint to see with the naked eye).

9) Now turn off all the constellation indicators. Select “Constellations |Boundaries” to turn off the boundaries, and “Constellations | Rey’s”(“Constellations | Stick Figures” in Starry Night Pro) to turn off thestick figures.

You learned in this chapter that another way to find an object is toknow its co-ordinates in the Horizon system: its altitude and azimuth.

1) Starry Night shows you the altitude and azimuth of the center of thescreen when you are scrolling around the sky. This is called your gazeinformation. In Starry Night Backyard, the gaze information appearsalong the bottom center of the screen. In Starry Night Pro, it willappear in one of the four corners, depending on how you have set upyour preferences. To pull up the gaze information at any time, justhold down the left mouse button.

29The Night Sky

2) Remember that altitude measures how far above the horizon an objectis located. Scroll up in the sky until the zenith (marked in red whenyou are scrolling or holding down the left mouse button)) is near thecenter of the screen. By looking at the gaze information, you will seethat the altitude is near 90 degrees. Similarly, if you scroll down in thesky so that the horizon is near the center of the screen, you will noticethat the altitude is near 0 degrees.

3) Azimuth is measured in degrees from North. If you scroll around sothat the compass marker for North is near the center of the screen andthen hold down the left mouse button, you will see that the azimuth isclose to 0 degrees (or just less than 360 degrees-because the sky isdivided into 360 degrees, the counter “resets” at 360). Likewise, ifyou move around on screen so that the East compass point is near thecentre of the screen, the azimuth will be near 90 degrees.

4) Now, we will use an object’s Alt/Az co-ordinates to find it in the sky.You are told that Sirius has an altitude of about 10 degrees, and anazimuth of about 124 degrees (remember that Alt/Az co-ordinates areonly valid for a specific time and will change as the star moves in thesky-these co-ordinates are valid for 10 PM on Dec.1, 2000 fromChicago). Use your understanding of how these co-ordinates relate tothe horizon and the direction markers to locate Sirius, the brighteststar in the sky. Remember that at any time, you can hold down the leftmouse key to find the Alt/Az co-ordinates of the center of the screen.

5) Starry Night can also tell you the Alt/Az co-ordinates of an object atany time. Open the Find dialog and find “Polaris”, also known as theNorth Star. Starry Night will then center on Polaris and identify it.Double-click on Polaris to pull up its information window (if nowindow opens, you didn’t have the cursor positioned directly overthe star. You should see the cursor icon change from a hand to anarrow before you double-click on the star). If you are using StarryNight Pro, click the “Position” tab. Make sure the dropbox in thiswindow reads “Local (Alt/Az)”. Polaris’s altitude and azimuth areshown just beneath the dropbox. Note that the altitude is approximatelyequal to your latitude (the latitude of Chicago is about 41.5 degreesN) and the azimuth is very close to 0 or 360 degrees. This is alwaysthe case for the North Star-it is the one star in the sky which doesn’tappear to move over time, so its Alt/Az co-ordinates remain constant.Close the Info Window.


6) Select “Guides | Celestial Poles” (“Guides| Equatorial | Celestial Poles”in Starry Night Pro). This will turn on markers for the North and SouthCelestial Poles. The North Celestial Pole has an altitude exactly equalto Chicago’s latitude, and an azimuth of 0. Note how close it is toPolaris. You may want to choose “Edit | Select None” (“Selection |Select None” in Starry Night Pro) to get rid of the label for Polaris,because it will overlap the label for the North Celestial Pole.

You will now use Starry Night to get a better understanding of thedifferent angular sizes of objects in the sky. Starry Night shows you a fieldof view approximately equal to what you can see with your eyes, about 100degrees. Your current field of view is always displayed near the two zoombuttons.

1) Starry Night allows you to find the angular separation of any twoobjects. Open the Find dialog and find “Alkaid”. This will center theend star in the handle of the Big Dipper.

2) Now position the cursor over this star. Wait for the cursor icon tochange from a hand to a pointer. Click and hold the mouse button andthen move the mouse. A line will appear on screen, drawn from Alkaidto the current position of your cursor. The angular separation of thetwo points is shown in degrees, arcminutes and arcseconds.

3) Continuing to hold the mouse button down, move the cursor over thestar at the end of the bowl of the Big Dipper, which is named Dubhe.You can see that the angular separation of the stars at the ends of theBig Dipper is just over 25 degrees. You can find the angular separationof any two objects this way.

4) Open the Find dialog and find “Andromeda Galaxy”. This will centeron the Andromeda galaxy. It is almost invisible in your current fieldof view. (make sure the “Magnify for best viewing” checkbox is stillturned off or the nebula will fill the screen).

5) Select “Guides | Field of View Indicators | 7 X 50 Binoculars”. Thisdraws a circle over the centre of the screen indicating the patch of skyyou would see if you were looking through binoculars of this commontype.

6) Use the zoom button to zoom in on the Andromeda galaxy. Zoom inuntil the circular outline covers almost the entire screen. Notice howthe field of view changes as you zoom in. Now you can see how the

31The Night Sky

size of the galaxy compares to the field you see through yourbinoculars. The binoculars have a field of view of about 7 degreesacross (the number shown near the zoom buttons will be a bit larger atabout 10 degrees, because it is the angular width from the left side ofthe screen to the right side). The angular width of the Andromedagalaxy is just under 2 degrees. You can see that the galaxy still appearsquite small when seen through binoculars of this type.

7) Press the square button to the right of the zoom buttons. This restoresyour field of view to 100 degrees.

8) Select “Guides | Field of View Indicators | 7 X 50 Binoculars” toremove the circular outline from the screen.

To conclude this exercise, you will learn a bit more about magnitudes.When you are looking at a regular 100 degree field of view, Starry Nightshows all stars brighter than magnitude 5.7 (5.5 in Starry Night Pro), whichmatches approximately what you could see in a dark sky. As you zoom in,the limiting magnitude automatically increases to show you dimmer objects.If you hold down the left mouse button, the limiting magnitude is shownalong the top of the screen.

1) Open the Find dialog and find “Sirius”.2) Bring up the info window for Sirius. The star’s magnitude is shown in

the window that appears (on Starry Night Pro, you need to make surethe “Info” tab in this window is marked) . For Sirius, the magnitude is–1.47.

3) By bringing up info windows for other nearby stars, you can see howmagnitude relates to brightness. For example, the three stars in Orion’sbelt (above Sirius, slightly to the right) all have magnitudes between1.6 and 2.3, about 3 to 4 magnitudes dimmer than Sirius, whileBetelgeuse (the bright star at the top of Orion) has a magnitude of 0.4,about 2 magnitudes dimmer than Sirius.

4) Changing the limiting magnitude affects the number of stars you see.Select “Sky | Small City Light Pollution” (“Sky | Light Pollution |Small City” in Starry Night Pro). The limiting magnitude is now 4.5.Note how many fewer stars are visible.

5) Select “Sky | Large City Light Pollution” (“Sky | Light Pollution |Large City” in Starry Night Pro). The limiting magnitude is now 3.0and only the brightest stars are visible.

6) Which of the three views do you think best matches the night sky younormally see? Select “Sky | No Light Pollution” (“Sky | Light Pollution| None” in Starry Night Pro), then turn on the constellation names andstick figures. Scroll around the sky and find Ursa Minor, also knownas the Little Dipper. The seven majors stars that make up the LittleDipper have a wide range of magnitudes. Polaris and Kochab are fairlybright with magnitudes around 2, while at the other extreme, Eta UrsaeMinoris has a magnitude of about 5. The other 3 stars have magnitudesin between these extremes. Find the magnitudes of all six stars (bybringing up their Info Windows) and write them down. The next timeyou are observing, determine which ones you can see with the nakedeye. Your limiting magnitude is somewhere between the magnitudeof the dimmest star in the Little Dipper that you can see, and themagnitude of the brightest star in the Dipper that you cannot see. Ifyou can see all the stars in the Little Dipper, count yourself lucky!

1.2

MOTIONS OF THE EARTH

There is movement in the sky. Not only do the sun, moon,

planets, comets, and asteroids move against the background of stars,but the sky itself moves. These changes happen because of the

motions of the earth as it spins on its axis and orbits the sun. As the

night passes and as the seasons change, we face different parts ofthe universe and see different stars and constellations.

Rotation of the Sky

Many celestial motions are too slow to be noticed over so short

a period as a night or even a month, but the nightly rotation of thesky happens on a scale that, with a little patience, we can experi-

ence while we gaze upward. The sky’s rotation is shown dramatically

in long time-exposure photographs centered on the North Star, whichshow the motion of stars as circular trails of different sizes centered

on the sky’s North Pole. We speak of the sky rotating overhead,

although we know that it is the earth that is turning. The earth makes


one rotation a day, spinning from

west to east, which causes the skyto turn from east to west. We speak

of the sun rising in the morning,

although we know that it is theearth that turns towards it, making

the sun appear to rise above the

horizon. The illusion is so convinc-ing that it wasn’t until the time of

Copernicus in the 16th century that

people accepted that the earth doesindeed turn on its axis.

Proving that the Earth SpinsHow would you demonstrate that the earthearthearthearthearth spins

rather than the skyskyskyskysky? No simple visual demonstrationexisted until the Frenchman Jean Foucault hung a massiveiron ball from the high dome of the Pantheon in Paris in1851 and set it swinging. Foucault demonstrated thatthis pendulum appears to slowly change the directionof its swing relative to the ground beneath. Since thependulum does not feel the orientation of the buildingit is attached to, the earth is free to rotate under itwithout affecting the direction of its swing. The pen-dulum feels the sum of the gravitational pull of the restof the universe and maintains a constant orientationrelative to the distant stars. Foucault Pendulums are foundtoday in planetariums and science museums.

Long time-exposure photoLong time-exposure photoLong time-exposure photoLong time-exposure photoLong time-exposure photoshowing star trails that arc asshowing star trails that arc asshowing star trails that arc asshowing star trails that arc asshowing star trails that arc asthe earth spins.the earth spins.the earth spins.the earth spins.the earth spins.

35Motions of the Earth

Annual Motion of the Sun

The daily rotation of the earth on its axis is one fundamental

motion of the earth (and of the sky). The second is the annual revo-lution of the earth around the sun.

Until the 16th century, it was taken as a matter of faith that the

earth does not move and is the center of all creation. Ancient Greekmusings contrary to this view were taken as mere philosophical

speculations. In 1543 the Polish astronomer Copernicus proposed

that the earth orbits the sun, rather than the other way around, but hehad no proof of what was to him a mathematical issue. Two

generations later the great Italian astronomer Galileo Galilei supplied

this proof in the form of telescopic observations of the phases ofVenus and the moons of Jupiter. He took up the “heliocentric” (sun-

centered) cause, but ran afoul of the authorities for his methods.

The truth was out, however, and by the mid-1600s it was univer-sally accepted (in Europe, at least) that the earth orbits the sun.

Long before anyone knew whether it was the sun or the earth

that moved, astronomers plotted the apparent path of the sun in thesky, relative to the background stars. This path is known as the

ecliptic, and it can be displayed in Starry Night. Astronomers also

noticed how the rise and set points of the sun on the horizon and itsnoon-time elevation varied with the changing seasons. They even

determined the length of the year – sometimes with surprising

accuracy.

The Zodiac

The sun’s path among the stars has been considered special

since it was first identified. The sun moves through certain constel-

lations, and even in the earliest times these constellations wereaccorded extra importance. The moon and planets pass through the


same constellations (plus several others – see “The ExtendedConstellations of the Zodiac” in chapter 3.3), and this also con-

tributed to their mystique. Although you cannot see the sun’s path

through the stars when you stand outside, Starry Night can show itto you.

The moon stays close to the sun’s path and provides a simpleway to divide it into segments. The moon travels around the sky a

little over 12 times while the sun travels around it once (this is the

long way of saying there are 12 months in one year; see chapter4.1). Rounding this to the convenient whole number 12 suggests

that the sun’s (and moon’s) path be divided into 12 segments, each

of an equal length (30°). Doing this links the motions of the sun andmoon at least symbolically.

Along the sun’s path are prominent groups of stars, like Scor-

pius and Gemini, and areas devoid of bright stars, like Aquariusand Cancer. The sun passes through 13 of the 88 constellations

mentioned earlier in its yearly journey through the stars. 12 of these

13 constellations are the classical constellations of the zodiac. Theywere all named by 600 BC, but most are far older. Scorpius, for

example, has been seen as a scorpion for at least 6,000 years, which

is long before the concept of the zodiac as a complete circle was

worked out. Most people associate the zodiac with astrology.

The moon and planets all stay close to the ecliptic line.


The 13th Constellation of the ZodiacThe constellation boundaries were arbitrary until

recently, and each astronomer (and astrologer) was freeto place the boundary lines where he saw fit. This causedendless confusion until 1930, when the constellationboundaries were fixed – among astronomers at least –by international agreement. (See “The Official Cygnus”in chapter 5.2) One effect of this tidying-up was to drawthe huge and ancient constellation Ophiuchus so that itintersected the ecliptic. The sun is actually within theboundaries of Ophiuchus from approximately November30 to December 17 each year (See chapter 3.3 to findyour astronomical birth constellation). In contrast, thesun is within Scorpius for only one week.

Annual Changes in the Stars

The sun’s apparent motion against the background of stars alsocauses seasonal changes to the constellations that we see at night.

Each day the sun is nearly 1° to the east, relative to the stars, of

where it was the day before. If we think of the sun as staying rela-tively still (and - after all - our timekeeping methods are based on

the position of the sun, rather than the stars), we can think of the

stars as moving westward 1° per day relative to the sun.Stars rise four minutes earlier each day, or 1/2 hour earlier each

week, or 2 hours earlier each month, or 24 hours earlier each year.

This is another way of saying that the cycle has been completed andthe stars rise at the same time again after one year has passed. If a

star rises at 2 a.m. on one date, it will rise at midnight one month


later, at 10 p.m. another month later, and at 8 p.m. yet another month

later (some stars near the North Star are exceptions to this rule, forthey are circumpolar, meaning that they do not rise and set, but

remain above the horizon all day and night). This is a handy rule of

thumb to remember when you are planning which stars and con-stellations to observe. If you have to stay up too late to see it now,

wait a few months and it will be conveniently placed in the evening

sky. This rule of thumb does not work for the moon or for Mercury,Venus, and Mars, as they have their own motion against the stars. It

is relatively accurate as a rule of thumb for Jupiter and Saturn (and

the outermost telescopic planets) and for most asteroids, becausethey orbit the sun very slowly and thus appear to share the same

motion as the stars.

Of course, the stars also set four minutes earlier each day, 1/2hour earlier each week, and 2 hours earlier each month. If Saturn or

Orion sets at 8 p.m. this month, it will set at 6 p.m. next month –

and you won’t see it. “What the sun giveth, the sun taketh away.”

Motions of the EarthStarry Night Exercise

This exercise will help you understand how the rotation of the Earthand the revolution of the Earth around the Sun alter the appearance of thesky. You should open Starry Night on your computer before beginning.

The Earth’s rotation on its axis is responsible for the rapid changes inthe position of the Sun, planets and stars in the sky. This example showswhy some stars are circumpolar.

1) Open a new window by choosing “File | New”.2) Select “Sky | Daylight” to turn sunlight off.3) Choose “Go | Viewing Location” to bring up the Viewing Location

window. Click the “Lookup” button and select Toronto from the listof locations. Click “OK”. Make sure that the box “DST” is not checkedand then press the “Set Location” button.


4) Set the time to 10:00 PM and the date to October 6, 1999. Scrollaround using the mouse so that you are looking north instead of south.

5) Open the “Find” dialog and type “Polaris” to center on the North Star.The Big Dipper is to the bottom left of Polaris, with its bowl almostparallel with the horizon. Note how the two “pointer stars” on the endof the Big Dipper’s bowl form a straight line which points to Polaris(make sure that your Starry Night window is full size so that you cansee the pointer stars and Polaris onscreen at the same time).

6) Set the time step in the time controls to 3 minutes, if it is not alreadyset to this value. Now press the Forward button to start time running.You can see that the stars appear to circle counter-clockwise aroundPolaris. Watch how the Big Dipper changes its orientation relative tothe North Star.

7) Notice how the speed at which a star moves is determined by its angularseparation from Polaris. The farther from Polaris a star is, the morerapidly it moves in the sky. Stop the time by pressing the Stop buttonin the time controls.

8) The angular distance of an object from Polaris also determines howlong a star stays above the horizon. Use the angular separation tool todraw a line from Polaris pointing straight down to the horizon (scrolldown a little if the horizon is not visible). How far is Polaris from thehorizon? You should find that it is about 43 degrees, which is also thelatitude of Toronto.

9) From Toronto, any star which has an angular separation from Polarisof less than 43 degrees is circumpolar, meaning it is always above thehorizon. Use the angular separation tool to measure the angularseparation between each of the stars in the Big Dipper and the NorthStar (if the Big Dipper is not visible, run time forward until it becomesvisible again). You will find that they are all less than 43 degrees,meaning that the Big Dipper is circumpolar as seen from Toronto.

This example shows you how the position of stars changes with theseasons. We say that Orion is a “winter constellation” in the NorthernHemisphere. Why is that?

1) Set the date to Dec. 21, 1999, and the time to 8:00 PM.2) Open the “Find” dialog and type “Betelgeuse” (watch the spelling!).

Betelgeuse is the bright star in Orion’s shoulder (note the 3 brightstars to its right which make up the belt). Betelgeuse has recentlyrisen above the horizon.


3) Right-click (click and hold on a Macintosh) on Betelgeuse to bringup a contextual menu. Select “Centre\Lock” from this menu

4) Now double-click on Betelegeuse to open its Info Window. Write downits rise and set times and then close this window.

5) Change the time step from 3 minutes to 1 day (choose the step marked“day”, not the one marked “day (sidereal)”. A sidereal day is a differentunit of time which is explained in chapter 3.1).

6) Use the Single Step Forward button to move time ahead by one day.Betelgeuse is now farther up and to the right relative to the horizon.Double-click on it again to find its new rise and set times. You shouldfind that they are both about 4 minutes earlier than the previous times.As you learned earlier, this is due to the Earth’s revolution about theSun.

7) Click the Forward (or Play) button to watch Betelgeuse’s location inthe sky change continuously. Stop time just before it falls beneath thehorizon. What is the date? You should find that it is near the beginningof June.

8) Click on the small outline of the Sun shown beside the time in theControl Panel. This should color the Sun yellow, meaning that DaylightSavings Time has been turned on.

9) Double-click on Betelgeuse to find its new rise and set times. Youshould notice that its rise and set times mean that it is now above thehorizon only during the day, so it will be invisible to us (rememberthat we turned off daylight for this example). But if you compare therise and set times with those you found for Dec.21, you will noticethat the total time which the star is above the horizon is exactly thesame: 13 hours and 2 minutes! So a star or constellation is classed as“winter” or “summer” not because it is above the horizon longer atthose times, but because it is above the horizon at night, when we cansee it.

10) Now change the time to Dec. 21, 2000. Click on the small outline ofthe Sun to turn off Daylight Savings Time. Double-click on Betelgeuseto find its rise and set times. You should find that they are the same asa year ago, as the Earth has returned to its previous place in the solarsystem relative to the sun.


To close this exercise, we will do something you can never do in reallife: watch the Sun move through the constellations of the Zodiac.Remember that this is caused by the Earth’s revolution around the Sun.

1) Set the time to 12:00 PM. We want to follow the Sun over the courseof a year, so we have to choose a time of day when it will always beabove the horizon.

2) Scroll around the sky to find the Sun, and then right-click (click andhold on the Mac) on it to bring up a contextual menu. Choose “Centre/Lock” from the menu.

3) Turn on the Ecliptic line by choosing “Guides | The Ecliptic” (“Guides| Ecliptic Guides | The Ecliptic” in Starry Night Pro).

4) Now turn on the Zodiac constellations by selecting “Constellations |Zodiac” ( in Starry Night Pro, you need to choose “Guides |Constellations | Zodiac Only” and then choose “Guides | Constellations| Stick Figures”). Turn on the constellation labels and boundaries aswell. The Sun should be in Sagittarius.

5) Make sure that the time step is still set to 1 day. Start time running bypressing the Forward (or Play) button in the time controls. If youfollow the motion for a year, you can see how the Sun moves throughall the constellations of the Zodiac, as well as moving through a partof the sky not in the classical Zodiac.

6) Stop time when the Sun is outside the boundaries of the Zodiac. Placethe cursor over the Sun. The screen will display the name of the Sunand the constellation it is in, which you will see is Ophiuchus.

This concludes the exercise for this chapter.


1.3

THE SOLAR SYSTEM

Motion of the Moon

Is the moon out tonight? Very likely – it is half the nights. If

not, it is probably out during the daytime. Many people are sur-prised to see the moon during the day, but it is visible in the day for

two weeks each month. Only for about three days each month when

the moon is new or nearly new can it not be seen at some time of theday or night.

The moon orbits the earth in counterclockwise direction as seen

from above the earth’s north pole. We see it move night by nightacross the sky from west to east. Each evening it is about 1/30 of

360°, or 12°, east of where it was the night before. It takes about 30

days to move through the constellations of the zodiac, and it spendsabout 2-1/2 days in each, on the average, before moving on to the

next.


In addition to moving around the sky, the moon changes its

shape. Planets move too, but the moon and rare bright comets arethe only naked-eye objects in the sky that change their shape day

by day. In the case of the moon, the change is quite predictable.

The chief principle is that the illumination that strikes the mooncomes only from the sun, and the sun lights up the side of the moon

that faces it. The sun lights up exactly one half of the moon at any

moment – but that is true for any ball lit by the sun. Hold a tennisball in sunlight, and no matter how you turn or position it, the half

facing the sun is the half that is lit. As the moon orbits the earth, the

side facing the sun remains lit. When the moon is new, the sidefacing the sun faces away from the earth, and we see the dark side

of the moon. This is not the same as the back side, which is the side

facing away from the earth; the moon’s back side and dark sidecoincide only at full moon.

The diagram above shows the Moon at eight positions on its orbit,The diagram above shows the Moon at eight positions on its orbit,The diagram above shows the Moon at eight positions on its orbit,The diagram above shows the Moon at eight positions on its orbit,The diagram above shows the Moon at eight positions on its orbit,along with a picture of what the Moon looks like at each positionalong with a picture of what the Moon looks like at each positionalong with a picture of what the Moon looks like at each positionalong with a picture of what the Moon looks like at each positionalong with a picture of what the Moon looks like at each positionas seen from Earth.as seen from Earth.as seen from Earth.as seen from Earth.as seen from Earth.

AAAAABBBBB

CCCCC

DDDDD

EEEEEFFFFF

GGGGG

HHHHH

AAAAA BBBBB CCCCC EEEEE GGGGG HHHHHDDDDD FFFFF

45The Solar System

As the moon moves out of alignment with the sun, it appears to

the east of the sun in our sky, and it sets after the sun. We now seemost of the dark side, as before, but we begin to see a thin crescent

of the lit side – we see a crescent moon in the evening sky. One

week after it is new, the moon has traveled one-quarter of the wayaround the earth and around the sky – and now it looks like a half-

moon. We see half of the lit side and half of the dark. After another

week the moon has moved another quarter of the way around theearth and is now opposite the sun, and we call it a full moon. We

are between the moon and sun and we see the moon’s lit side. Now

– and only now – is the moon’s back side the same as its dark side.After another week, the moon has moved three-quarters of the way

around the earth and is in a similar position to when it was at first-

quarter, but now the left side of the moon is illuminated, rather thanthe right. It looks like a half moon. During the final week of the

lunar month it is a crescent again that grows progressively slimmer.

During the period between first and last quarter, the moon’sphase is called gibbous. It is a waxing gibbous between first-quarter

and full and a waning gibbous between full and third quarter (waxing

and waning are from Old English and mean “increasing” and“decreasing” respectively). From new to first quarter it is a waxingcrescent, and between third quarter and new a waning crescent.

Eclipses

A solar eclipse happens when the moon moves in front of thesun and blocks the sun’s light. The eclipse can be either partial (the

moon does not completely cover the sun and blocks only part of it)

or total (the moon completely covers the sun). A partial eclipse canbe annular if the moon moves directly in front of the sun but is not

large enough to completely cover it, leaving a ring of sunlight


(“annular” comes from the Latin word for ring) shining around the

edge of the moon. Annular eclipses exist because the orbit of themoon around the earth is an ellipse, not a circle. This means that the

moon is not always the same distance from earth. When it is farther

from earth than normal, it does not appear as large in the sky.Therefore, it cannot completely cover the sun and we see an annular

eclipse.

Only during a total eclipse does the sky grow dark and thesun’s dazzling corona burst forth in radiant glory. By an accident of

the earth’s history, our sun is about 400 times larger than our moon,

but also 400 times farther away. This means that both objects appearto be almost exactly the same size. For this reason, total eclipses

Observing Solar EclipsesThe sun’s surface is too bright to look at without

risking serious eye damage, and this remains true evenwhen most of the surface is covered by the moon. Al-ways use proper precautions when observing the sun,both when watching an eclipse and when looking forsunspots. Use an approved dark or mirrored filter placedin front of your eye or in front of your telescope’s mainlens or mirror (do not use a filter that screws into youreyepiece) or project the sun’s image onto a white sur-face. Do not use home-made filters which may block thesun’s visible light but allow harmful amounts of infraredor ultraviolet light to pass through. You can purchasesolar filters from a variety of sources.

Only during the brief totality phase of a total eclipseit is OK to look at the sun without filters – and youshould, to enjoy the spectacle!

47The Solar System

are very brief, because the moon and sun have to be lined up almost

exactly, and the motion of the moon prevents a total eclipse fromlasting more than a few minutes.

During each eclipse, a similar sequence of events happens. Open

the file “SunEclipse1” to follow the course of a partial eclipse asseen from San Diego, California, in October 2014. The moon appears

as a pale disk near the sun. The sun will appear to stand still while

the moon approaches, eclipses it, and then moves on. First contactis the moment when the moon’s edge first “touches” the sun and the

eclipse begins – in this eclipse it is at 1:15 p.m. Second contact is

when the moon leaves the sun and the eclipse ends; in this case it isat 3:42 p.m. As seen from San Diego, the moon never covers more

than a third of the sun’s face.

For total and annular eclipses there are four stages. Open thefile “SunEclipse2” (an eclipse widely seen from Mexico in 1991).

First contact is at 10:43. Second contact now becomes the moment

when the moon first completely covers the sun; it is at 12:10 p.m.The eclipse is total between second and third contacts, and totality

lasts only a few minutes (about 7 minutes for this eclipse). The eclipse

ends at fourth contact (1:39 p.m.). You may wish to view the momentof totality again, in slower motion. Stop time and change your time

step to 1 second. Then change the time to about 12:08 p.m. before

starting time running forward again.Annular eclipses follow a similar sequence except that between

second and third contacts the moon is silhouetted against the sun,

which appears as an unbroken ring for a few minutes. Open the file“SunEclipse3” to view an annular eclipse as seen from near

Oklahoma City in the spring of 1994. Notice how the sky does not

darken completely, even though the sun is almost completely covered


by the moon at the eclipse’s peak.

The precise times and circumstances of a solar eclipse dependon your observing location. You stand in the shadow of the moon

during a solar eclipse, and the central part of the moon’s shadow

(the umbra) is no more than 269 km (167 miles) wide when it reachesthe earth. If you are within it, the eclipse is total, but if you are not,

it is partial or you miss it entirely.

The moon is moving and so is its shadow. The moon’s shadowsweeps over the earth at about 1600 km (1,000 miles) per hour,

racing from west to east. Where the shadow first strikes the edge of

the earth, observers see the sun (which is directly behind the moon)low in the east; it is a sunrise eclipse. People at the center of the

shadow experience the eclipse at noon, and people at the eastern

edge of the earth see it at sunset. Although the path of totality (or

A Solar Eclipse From the MoonPeople on the moon during a total solar eclipse

would see nothing unusual happen to the sun, but theywould see the shadow of the moon race across the earth.It would resemble a lunar eclipse as seen from earth,but the moon’s shadow is far smaller than the earth’sshadow and it appears as a dark splotch with a blackcenter that moves across the surface of the earth.

Open the file “SunEclipse4” to see the famous July1991 eclipse from the moon. Everywhere within the areaof the large shadow experiences a partial eclipse whilethe tiny black dot in the middle of the shadow marksthe path of totality. Zoom in if you want a closer look atthe area of totality.

49The Solar System

annularity) is narrow and comparatively few people see a total or

annular eclipse, the outer part of the moon’s shadow is wide andmuch of the earth’s population sees a partial eclipse at the same

time.The solar eclipse has a less-spectacular counterpart in the lunar

eclipse, caused by the earth passing between the sun and the moon.

Lunar eclipses are described in more detail in section 4.1.

Planets

While the stars appear in the same configuration relative toeach other every night, the planets do not. Much like the sun, they

move slowly through the sky from constellation to constellation.

Some planets move much faster than others. All the planets movethrough the same constellations – more or less — as the sun. This is

because the planets all move around the sun in nearly the same plane,

orbiting above the sun’s equator. From earth, we see this plane edge-on and it appears to us as a line called the ecliptic. The sun always

remains on this line and the planets never stray far from it.

Although the sun, moon, and planets move across the sky andit appears that we are at the center of all we see, we know now that

this is not the case. The sun is at the center of our solar system, and

the planets (including the Earth) circle around it counterclockwiseas seen from above the Sun’s north pole. The planets all move in the

same direction, but not at the same speeds. The closer a planet is to

the sun, the faster it moves because it feels the pull of the sun’sgravity more strongly. Planets near the sun also have shorter paths

and complete their years more quickly; Mercury orbits the sun in

only 88 earth days but Pluto takes 248 earth years. Each planet travelsat a nearly uniform speed on a nearly circular orbit. (Mercury and

Pluto are the only noticeable exceptions to these last two, but they


follow the general rule of distance by moving fastest when closest

to the sun.)Perhaps the most profound observation – and one that would

have astounded all ancient people – is that the earth is one of theplanets and moves with and among the others. We view the skyfrom a platform that is itself in motion.

The Solar SystemStarry Night Exercise

After working through this exercise, you will know how to use StarryNight to learn more about the motions of the Moon and planets.

We begin by watching the motion of the Moon.1) Open a new window by choosing “File | New”.2) Turn off daylight by selecting “Sky | Daylight”.3) If it is not already open, open the Planet List by selecting “Window |

Planet List” (“Window | Planets” in Starry Night Pro).4) Click on the name of the Moon in the Planet List, and then click the

Lock icon. This should center the Moon. If a window comes up sayingthe Moon is beneath the horizon, click the “Reset Time” button (the“Hide” button in Starry Night Pro) in this window.

5) Two fields in the Info Window can tell you about the Moon’s age.Double-click on the Moon to open the Info Window. “DiscIllumination” (just called “Illumination” in Starry Night Pro) tells youwhat percentage of the moon’s face is illuminated, ranging from 0%at new moon to 100% at full moon. “Age” tells you how long it hasbeen since the Moon was last new, and also gives the phase name.The moon rise and set times are also shown (On Starry Night Pro, youneed to click the “Position” tab to see this information). Close theInfo Window.

6) Turn on the Zodiac constellations and the Ecliptic line..7) Change the timestep to 1 day.

51The Solar System

8) Press the Single Step Forward button to watch how much the Moonmoves in a day. Keep pressing the Single Step Forward button to watchthe phases change (if the moon falls beneath the horizon, continueclicking the Single Step Forward button until it reappears). Double-click on the Moon at any time to find out its age and phase. Note howthe moon stays close to the ecliptic, always remaining within about 5degrees.

We will move on to study the motion of the planets.1) Select “Labels | Planets/Sun” (“Sky | Labels | Planets | Sun” in Starry

Night Pro) to turn on labels for all the planets. Scroll around the skyand notice how all planets above the horizon are near the line of theEcliptic.

2) In the Planet List, click on Jupiter’s name and then click the Lockicon. If Jupiter is beneath the horizon, click the “Reset Time” button(“Hide” button in Starry Night Pro) in the window which comes up.

3) Start time flowing by pressing the Forward (or Play) button (if Jupiterfalls beneath the horizon, just continue to let time flow until Jupitercomes back above the horizon). You will see that Jupiter also movesthrough the Zodiac constellations, but much slower than the Sun did.To see why this is so, we need to change our viewpoint to see themotion of the planets from a new perspective.

4) Stop the time.5) Choose “Go | Viewing Location”. Select “Sun” from the drop-box

near the top of the Viewing Location window which appears. Thenchange your latitude to 90 degrees N and press “Set Location”. Youare now on the north pole of the Sun!

6) Click on the Sun’s name in the Planet List and then click the lockicon. You are now looking back down on the Sun.

7) Click the arrow in the Elevation box. This box is right near therocketship buttons and should read “3m” intially. Select 1 ly from thelist of elevations which appear in the menu. You are now 1 light yearabove the Sun.

8) Choose “Go | Viewing Location” again. In the window which comesup, check the box marked “Hover”.

9) In the Planet List, turn on the orbits of every planet. You turn on aplanet’s orbit by clicking in the column along the right side of thePlanet List beside each planet’s name (in Starry Night Pro, you clickon the leftmost of the 3 columns). You won’t be able to see the orbitsyet, because you are so far above the solar system.


10) To make sure all the labels will show up, select “Labels | LabelSettings” (in Starry Night Pro, choose “Sky | Labels | Label Options”).This brings up the Label Options window. On the right side of thiswindow, click on the name “Planets” (don’t click on the checkboxbeside the name- it should already be checked). On the right side ofthe Label Options window is a checkbox marked “Label Even WhenVery Dim”. Make sure this box is checked.

11) Now zoom in on the sun by using the zoom buttons. Keep zooming inuntil the orbit of Saturn almost fills the screen.

12) Change the timestep to 10 days. Press the Forward button to starttime running. You can see how quickly the inner planets movecompared with Saturn and Jupiter. The outer planets of Uranus,Neptune and Pluto move even slower. This is why Jupiter movesthrough the Zodiac so slowly.

This concludes the exercise for this chapter.

53Skywatching

Section 2

Observational Advice

STARRY NIGHT is a great tool for learning

more about astronomy, but it will never

replace stepping outside and observing the

wonders of nature with your own eyes. This

section gives you some help on getting more

out of your observations. Chapter 2.1 gives

general hints & tips, as well as brief advice on

buying binoculars or a telescope. Chapter 2.2

explains the different astronomical co-ordinate

systems, which you will use to help you locate

objects in the sky.

2.1

SKYWATCHING

Hints & Tips

Although casual skywatching takes no special training or skills,

there are some things to keep in mind that will improve theexperience.

First, as often as possible, observe away from city lights.

Through all of history – until this century – the sky was dark atnight even in the world’s major cities, and (ignoring chimney smoke)

the stars shone as brightly over New York City as over a rural Kansas

town. Our modern cities are bathed in a perpetual twilight from themillions of lights that illuminate the air itself, not to mention any

dust, smoke, and smog in the air, and this lets us see only the brighter

stars. Astronomers refer to this unwanted stray light as light pollutionand some campaign against inefficient lighting. Children who grow

up in the city do not know what the stars look like and have no

Section 2 – Observational Advice56

appreciation for the beauty of the sky, and even adults often forget.

It is common to hear an audience gasp and even applaud when thestars come out in an urban planetarium show.

People are most aware of the poor quality of an urban sky when

a bright comet appears and they are told by the news media to “goto a dark site” to see it properly. Even if a comet can be seen from

within a city, it is a pale shadow of what people see from a dark

location.Many people conduct their most rewarding stargazing while

on vacation. The national parks and national forests are generally

exceptionally dark at night, and the sky becomes an attraction oncedarkness has settled over the trees, meadows, and waterfalls. Take

binoculars or a spotting telescope on your next vacation and continue

to sightsee after sunset.A different type of light pollution is caused by our moon. Even

the darkest locations suffer when the moon is bright. For two weeks

each month centered on the date of the full moon, moonlight bathesthe sky in its own whitish twilight, reduces contrast, and obscures

faint objects. There is nothing one can do about moonlight other

than to try to work around it. During the week before full moon, themoon sets before sunrise and the sky is fully dark only before

morning twilight begins; during the week after full moon, the sky is

dark briefly between the end of evening twilight and moonrise.Another observing consideration is comfort. In most places it

is cold in winter, especially at night, and winter stargazing means

keeping warm. It is one thing to shovel snow or walk briskly to thestore on a cold day; it is quite another to stand motionless and watch

Orion in the dark. Your body generates little heat while watching

the stars, and most people must dress warmer than anticipated. Thisis often true even in the summer, when it cools off at night. Padded

footwear is important in the winter if you will be standing on cold

ground. Many people watch meteor showers from the comfort of a

57Skywatching

sleeping bag on a lawn chair or ground pad. Bugs can be a problem

too. Bug spray is an essential accessory for many summer stargazers(but be sure to not get spray on your optics!).

Another useful skywatching accessory is a red flashlight.

Astronomy specialty shops sell observers’ flashlights that shine witha very pure red light, but you can adapt a conventional flashlight by

fitting a few pieces of red gel purchased at an art supply store over

the lens. If you want to run Starry Night outdoors on a laptop duringyour observing sessions, you may also wish to make a red gel cover

to fit over your computer screen (unless you own Starry Night Pro,

in which case you can just turn on its night vision mode).Using a red flashlight preserves your night vision. Step out-

doors from a brightly lit room and notice how long it takes your

eyes to adapt to the dark. After a few minutes your pupils will adjustto the lower light levels, and you will be able to see more stars.

Turn on a white flashlight and look at a printed star chart. When

you look up at the sky again, you will notice that your enhancednight vision is gone. Once you have regained it, look at the same

printed chart with a red flashlight and then look back at the sky to

appreciate the improvement. Using red lights to preserve night visionis a familiar trick to submariners and others who may have to

suddenly leave a lit room and step outdoors ready to see in the dark.

Binoculars and Telescopes

Your eyes are a perfectly fine tool for stargazing, and there ismuch to see with them alone on a dark night. But all sky watchers

yearn to see more and fainter objects and to see them better, and

eventually they want to buy a telescope or pair of binoculars. At theGriffith Observatory in Los Angeles, we recommend that people

on a budget (or those who are very concerned with portability) start


with a pair of binoculars. Binoculars are under-appreciated and are

wonderful for many kinds of casual stargazing, and even experi-enced observers enjoy having a pair of good binoculars for wide-

field views of the Milky Way (and for their portability!). You

shouldn’t leave home without a pair.Binoculars have the advantages of being relatively inexpen-

sive, highly portable, easy to use, and they give the best views of

the Milky Way (and often of bright comets). Plus, you can use themfor non-astronomical sightseeing, especially while on vacation.

Expense is relative, and in general you get what you pay for.

Do not be tempted by $50 binoculars at discount camera stores anddrug stores. They use plastic parts (including lenses!), are not well

assembled, and should only be reserved for situations where they

are at risk (such as boating, or perhaps for use by an accident-pronechild). Anticipate spending $150 or more for a decent pair that will

give good color-corrected images and that will provide a lifetime of

use and enjoyment. A good pair of binoculars can be enjoyed fordecades and handed down to the next generation.

Astronomers have different requirements from sportsmen, and

their choice of binoculars is somewhat different. Adequate light isseldom a problem at the horse races, but it certainly is when peer-

ing at the Milky Way, so astronomers want binoculars with large

lenses. The lens size is expressed as its diameter in millimeters, andit is the second number that characterizes a pair of binoculars.

Astronomers want 50 mm lenses (or larger), and they use smaller

binoculars only when size and weight is critical, as when back-packing. Larger binoculars with 70 or 80 mm lenses are great for

astronomy, but they are expensive and they must be used on a tripod.

The other characteristic of binoculars is the magnification, whichappears as the first number. Magnification is not so important, and

in any case comes within the range of 7 to 12. 10-power is a popular

compromise, although it is hard to hold them steady without a tripod.

59Skywatching

The best all-purpose binoculars for stargazers, then, are 7X50 to

10X50; seven to ten power with a lens diameter of 50 mm.Test before buying. If you wear eyeglasses to correct for distance

but have no significant astigmatism, use binoculars without glasses.

Check before buying that they will focus at infinity to your sightwith your glasses off. You probably won’t be able to focus on an

object which is far enough away while you are inside a store. Ask to

test them outside and focus on a distant tree or building. Some peoplewho normally wear glasses use contact lenses when observing,

especially to avoid problems when switching back and forth between

the sky, a finder scope, binoculars, the eyepiece, and star charts.One step up from binoculars – and a good starter telescope – is

Light Collecting AreaA 50mm binocular lens does not sound much larger

than a 35mm lens, and a 20 cm mirror does not soundmuch larger than a 15cm mirror, but in both cases thelarger is twicetwicetwicetwicetwice the size of the smaller – if you arecomparing the light-collecting area. A large lens or mirrorcollects more light than a small one and reveals fainterobjects, and this is vitally important in astronomy.Compare the surface area, not the diameter. The area ofa circle is proportional to the length of the diametersquared (the diameter times itself ), so a small increasein diameter makes a big difference in area. The ratio ofthe lens area of 50mm and 35mm binoculars is 502/352

= 2500/1225, or two to one; similarly, the ratio of a20cm to a 15cm telescope mirror is 400/225 or almosttwo to one.


a “spotting telescope”. These telescopes are easy to use and feature

low magnification and a wide field of view. While standardbinoculars have a magnification of 12X or less, a spotting telescope

will reach 40 power or more, giving great views of the moon and

bringing in the moons of Jupiter, the rings of Saturn, and manydouble stars. They’re great for watching wildlife as well. Their name

comes from their use “spotting” the hits in target practice, but many

amateur astronomers have a spotting telescope for vacations andtravel. Prices range from $100 to well over a $1000 for a 70 – 100

mm spotting telescope with eyepieces, tripod, and carrying case or

bag.Recommendations for purchasing an astronomical telescope

are beyond the scope of this book, but here are a few hints.

Amateur telescopes come in three flavors: refractors, whichuse a lens; reflectors, which use a mirror; and hybrids called Schmidt-

Cassegrains. All telescopes also use eyepieces to magnify images.

Refractors are easier to make in small sizes and they dominatethe market in the lowest price range; they are what you see in toy

stores. Reflectors are more complex and are seldom sold for less

than $200. Larger refractors with a lens 8 cm (3 inches) in diameteror larger can be superb instruments, but they are more expensive

than a reflector of the same size and are starter telescopes only for

the rich and famous. Reflecting telescopes withmirrors in the 15 to 20 cm (6- to 8-inch) range

are the workhorses of amateur astronomers.

Reflecting telescopes with the Dobsonian-style mounting have become popular recently

and are a good buy with prices starting at about

$500; they are also relatively easy for hobbyiststo build using common workshop tools and

following published plans. Later you can work

up to (or build) telescopes with apertures ofDobsonian-styleDobsonian-styleDobsonian-styleDobsonian-styleDobsonian-stylemountingmountingmountingmountingmounting

61Skywatching

14, 20, or more inches! Schmidt-Cassegrain telescopes are compact

and portable but more expensive for a given size than simplereflectors.

Know where to shop – and where not to shop. Avoid discount

camera stores (the kind that are always going out of business),department stores, and toy stores. Stay away from telescopes

promoted on the basis of their magnification. A “464-power!”

wonder is certainly a case of false advertising at best. To avoiddisappointment, patronize telescope stores (which unfortunately are

few and far between), the better camera stores, and catalogs.

Do your homework before you put your money down. Readthe magazines “Sky & Telescope” and “Astronomy” (sold at major

newsstands). They regularly print review articles on telescopes and

other equipment for amateur astronomers, and their advertisers area great source of free information. Request, read, and compare

catalogs. Mail-order telescope sales are big business and make sense

for people who live far from cities with proper telescope stores.Find out if there is a local astronomy club, and if so, attend a meeting.

The club’s members will be a wonderful source of practical

information and recommendations. Clubs are also a source of goodused equipment. A local planetarium or science museum can also

offer advice. Some have large gift shops and sell telescopes and

accessories. Last but not least, there are many excellent Internetresources to help you with your search. The major telescope

manufacturers maintain sites with extensive information on their

products, and there are also many independent sites which comparethe merits of different telescopes. Check the web site

www.livesky.com for links to some of the best sites.

No matter what type of telescope you choose, it is good to haverealistic expectations of what you will see through your new

instrument. It won’t perform like the Hubble Space Telescope! You

won’t see the colorful images found in astronomy books and


magazines; such photographs are long time-exposures taken with

professional telescopes. But you will see the real, genuine thing –and that personal experience is worth a lot.

2.2

CO-ORDINATE SYSTEMS

Whether you are looking through your new telescope or just

gazing up with the naked eye, you’ll need to have a good under-standing of the different astronomical co-ordinate systems to find

many of the more interesting jewels in the night sky.

What is north? … east? … south? … west? These directionsare astronomical. The sky’s rotation defines directions on the ground,

positions on the earth, and the main astronomical co-ordinate system

used to specify positions in the sky. In section One, the Altitude-Azimuth co-ordinate system was described. We saw that any object

in the sky could be uniquely identified by its angular height above

(or below) the horizon and its compass direction. This is called a“Horizon” or “Local” co-ordinate system because an object’s co-

ordinates in this system are valid only for one location and only for

one time. Astronomers use other more general co-ordinate systemsto describe the positions of objects in the sky.

Of course, it is the earth’s rotation that determines directions

and co-ordinates, but when the co-ordinate system was set up more


than 2100 years ago it was almost universally “known” that the

earth is spherical but stationary while the sky turns overhead. Theperson who invented the system of using longitude and latitude to

specify positions on the earth was the Greek astronomer and mathe-

matician Hipparchus. He rejected the notion that the earth rotateson its axis, but his solution works just the same.

The earth’s rotation determines the position of the earth’s two

poles and equator. The poles are where the earth’s rotation axis pene-trates the earth’s surface as it continues from the earth’s center

infinitely far into space, and the equator is the line midway between

the poles. Lines of longitude, called meridians, run from pole topole and divide the earth into segments that specify a point’s distance

east of the zero segment, which last century was arbitrarily chosen

to be the longitude of the Greenwich Observatory in England (thePrime Meridian). Longitude is measured either in degrees, in which

case there are 360° around the earth’s circumference, or in hours,

minutes, and seconds, which represents the actual rotation of theearth. If you live 7 hours west of Greenwich, a star is overhead at

your location 7 hours after it is overhead at Greenwich. There are

15° in one hour of longi-tude, which is the amount

the earth rotates in one

hour of time. Hours aresubdivided into minutes

and seconds, and as you

learned in chapter 1.1,degrees are subdivided

into arcminutes and arc-

seconds. Longitudes in

Starry Night are

expressed in degrees east Earth’s lines of latitude and longitudeEarth’s lines of latitude and longitudeEarth’s lines of latitude and longitudeEarth’s lines of latitude and longitudeEarth’s lines of latitude and longitude

65Co-ordinate Systems

or west of Greenwich, while latitude is the angular distance of a

location north or south of the earth’s equator, expressed in degrees.The latitude of the equator is 0°, of the north pole 90° N or +90°,

and of the South Pole 90° S or –90°.

Positions on earth are expressed as their longitude and latitude,in that order, which is the same as the angular distance west (or

east) of the Prime Meridian passing through Greenwich and the

angular distance north or south of the equator.There is a corresponding system which astronomers univer-

sally use to specify positions on the celestial sphere. It is called the

equatorial co-ordinate system. The celestial sphere is a concept,not a real object, but it is convenient to think of the globe of the sky

as an invisible crystalline sphere that has the stars attached to it,

with the sun, moon, and planets moving along its surface (it is anactual aluminum sphere in a planetarium theater). Just as positions

on earth are expressed by:

1) the time it takes the earth to rotate from the zero meridian atGreenwich to the specified location,

2) the angular distance north or south of the earth’s equator,

so celestial positions are expressed in terms of:1) the time the sky takes to rotate from an arbitrary zero

meridian to the specified point,

2) the angular distance north or south of the sky’s celestialequator.

The two values are called right ascension and declination re-

spectively, abbreviated RA and Dec. Values in right ascension areexpressed in hours, minutes, and seconds; values in declination are

expressed in degrees, arcminutes, and arcseconds. For example, the

equatorial co-ordinates of Rigel would normally be expressed inthis format: 5h 14m, -8° 12’.

To determine an object’s right ascension, we first need to choose

a line of 0h RA. Much like the line of 0° longitude on Earth is the


Prime Meridian, the line of 0h Right Ascension is known as the

Celestial Meridian. This should not be confused with the localmeridian, which is an imaginary line in the sky running from north

to south through the zenith. Which stars the local meridian passes

through depends on your location, whereas the celestial meridianalways passes through the same stars and constellations.

Defining the celestial meridian is a two-step process: we first

define the vernal equinox as one of the two points on the celestialsphere where the celestial equator meets the ecliptic (the vernal

equinox is also the moment in time when the sun crosses the celestial

equator in March; it is the moment when spring begins in the earth’snorthern hemisphere – see Chapter 3.2). The celestial equator is

the projection of the earth’s equator into space, and the ecliptic is

the sun’s apparent annual path around the sky.Open the file “Intersection” to see how the vernal equinox is

one of the two points where the celestial equator and the ecliptic

intersect (the second point is called the autumnal equinox, and it isbeneath the horizon). Start time running forward to see that the vernal

equinox moves in the sky over the course of a night just like a star.

It therefore has a fixed place in the celestial sphere, in the con-stellation Pisces.

Having defined the vernal equinox, the celestial meridian is

then defined as the line extending from the North Celestial Polethrough the vernal equinox to the South Celestial Pole.

Once a meridian has been defined, an object’s right ascension

is determined by its distance from this meridian. An object with anRA of 12 hours has the same RA as the autumnal equinox, while

objects with an RA of 6h or 18h are halfway between the vernal and

autumnal equinoxes.Declination is perhaps easier to understand than right

ascension, because its reference line is the celestial equator, which

is just the projection of Earth’s equator onto the celestial sphere. It


is therefore an exact counterpart to latitude on Earth. We measure

declination in degrees north or south of the celestial equator, whichitself has 0° declination. The declination of the North Celestial Pole

is +90° or 90° N and the South Celestial Pole is –90° or 90° S.

The equatorial co-ordinates of a star do not change as the hourspass (Actually, this is not precisely true because of proper motionand precession, but these motions are very slow, and do not change

the position of a star enough to notice, at least with the unaided eye,during a human lifetime. See chapters 3.3 and 5.2 for more on proper

motion and precession, respectively.) Just as the longitude and

latitude of Boston do not change as the earth turns, the equatorialco-ordinates of Rigel remain constant night after night and year

after year. The equatorial co-ordinates of a planet or comet do change

from night to night as the planet or comet moves across the skyrelative to the stars beyond.

It is very useful to have a feeling for the equatorial co-ordinate

system if you wish to be able to find things in the sky. An invalu-able exercise

is to stand

outside atnight and

imagine the

co-ordinatesas if they

w e r e

chalked onthe sky, and

to imagine

how theyturn with the

c e l e s t i a l

sphere as it

celestial equatorcelestial equatorcelestial equatorcelestial equatorcelestial equator

eclipticeclipticeclipticeclipticecliptic


wheels overhead. With Starry Night, you don’t have to imagine –

just choose “Guides | Celestial Grid” (“Guides | Equatorial | Grid”in Starry Night Pro) and let time run forward. The equatorial co-

ordinate system is based on the rotation of the earth and is most

useful for specifying the position of an object on the celestial spherein relation to the earth (to the earth’s equator and poles). A third

system, called the ecliptic co-ordinate system, is more useful when

you want to specify the position of an object relative to the sun andto the orientation of the solar system.

The counterpart of the equator in the ecliptic co-ordinate system

is the ecliptic, which is the earth’s path around the sun. It is also thesun’s apparent annual path around the sky – another way of

expressing the same concept. Positions in the ecliptic co-ordinate

system are specified in ecliptic longitude and ecliptic latitude, bothin degrees. The ecliptic itself has an ecliptic latitude of 0°, and the

ecliptic poles have ecliptic latitudes of +90° and –90°. The North

Ecliptic Pole – the north pole of the solar system – is in Draco, andthe South Ecliptic Pole is in Dorado not far from the Large Magellan-

ic Cloud. Positions along the ecliptic are measured eastward in

degrees from an arbitrary starting point, which is again chosen tobe the vernal equinox – the point where the ecliptic intersects the

celestial equator in Pisces. Ecliptic longitudes are numbered between

0° and 360°.Specifying an object’s position in ecliptic co-ordinates tells you

when the sun is closest to it and how far it is from the sun’s path.

The ecliptic position of Antares, for example, is 250°, -5°; the sunis closest to longitude 250° on December 2 (you have to look up the

date – or let Starry Night show you), and at that time Antares is 5°south of the sun. Ecliptic co-ordinates are almost exclusively usedfor solar system objects. Planetary conjunctions are usually

expressed in ecliptic co-ordinates (see the section on conjunctions

in Chapter 4.2). When Apollo astronauts left the earth’s vicinity to


travel to the moon, they left the earth’s co-ordinate system behind

and used the ecliptic system to navigate. Special ecliptic-based starcharts were prepared for their voyages.

A fourth co-ordinate system is of specialized use to people in-

terested in the Milky Way. It is the galactic co-ordinate system, andit uses the equator and poles of the Milky Way Galaxy as its funda-

mental plane and polar points. It lets you specify the position of an

object in an “east-west longitude” sense relative to the center of theMilky Way and in a “north-south latitude” sense relative to the plane

of the Milky Way. The Milky Way’s center is in Sagittarius (at co-

ordinates 17h 46m, -29° in the equatorial system) and its north andsouth poles are in Coma Berenices and Sculptor (at co-ordinates

12h 51m, +27° and 0h 51m, -28°) respectively. The system lets you

specify a star’s position as its galactic longitude and galactic lati-tude. Only Starry Night Pro can display the galactic and ecliptic

co-ordinate systems.

There are many other less well-known co-ordinate systems,such as those associated with other planets or moons. These co-

ordinate systems are used by solar-system astronomers when

calculating planetary spacecraft trajectories and other phenomenaassociated with particular planets.

The most important co-ordinate systems for finding your way

around the sky are the Alt-Az (horizon) and equatorial co-ordinatesystems. The co-ordinates you will find in a book are usually equa-

torial, but you need the horizon co-ordinates to actually know where

in the sky you should be looking! Starry Night offers an easy wayto get the different co-ordinates of any object. Just double-click on

the object to bring up an Info Window. There is a drop-down menu

which lists the four co-ordinate systems we have discussed (thereare actually two different options for equatorial co-ordinates shown.

The difference between these two is discussed in Chapter 3.3). When

you select one of these options, the appropriate co-ordinates show


up beneath this pop-up menu. This makes it easy to convert between

different systems. Remember that horizon co-ordinates are time-dependent, so make sure to set the time in Starry Night to the time

at which you want to observe. Once you have a good understanding

of co-ordinate systems, you will spend less time figuring out whereto look, leaving you more time for the observations themselves!

Section 3

Earth’s Celestial Cycles

AS WE SAW in section 1, there are several

sources for the apparent motions of the stars,

planets and other objects that we see in the night sky.

We can divide these motions into two categories: the

actual motion of the objects, and the apparent motion

caused in reality by the earth’s movements. This

section of Starry Night Companion looks at the earth’s

movements. The apparent motion due to the earth can

be subdivided into three distinct components: rotation,

revolution and precession. These sources of

motion are all periodic, meaning that their

effect is repeated after a certain

time. However, the periods for

the three motions are

radically different: the

earth completes one

rotation in a day, one

revolution of the sun

in a year, and one

precession cycle on its

axis in 26 000 years!

Rotation is covered in

chapter 3.1, revolution in

chapter 3.2, and

precession in chapter 3.3.

3.1

ROTATION

What’s in a Day?

The time it takes the earth to spin once is a day, but how long is

that? Twenty-four hours is the fast and simple answer, but it is justone of several that are correct. “Relative to what?” is the second

half of the question.

Our lives are regulated by the appearance (and disappearance)of the sun, and the solar day is the fundamental unit of time. The

average time from one sunrise to the next is one solar day, and it

does equal exactly 24 hours. That is the time it takes the sun, onaverage, to return to the same position relative to you and your

horizon (“on the average” is an important qualification because the

length of the day changes with the seasons; see Chapter 3.2 forseasonal changes in the sun’s motion).

Section 3 – Earth’s Celestial Cycles74

We could also think of the time it takes the earth to spin once

relative to the stars. Because the sun moves eastward a slight distancerelative to the stars each day, during a sidereal day (“sidereal” means

“stellar”) the earth turns less than once relative to the sun while it

turns exactly once relative to the stars. This makes a sidereal dayshorter than a solar day by four minutes. The sidereal day is 23

hours 56 minutes and 4 seconds long.

Open the file “Day” and notice that the sun is on the meridian– the imaginary line that runs from due south to straight overhead

and on to the north horizon. Daylight is turned off so that we can

see the stars as well as the sun. Some stars near the meridian arelabelled.

Click the Single Step Forward button to advance the time by

one sidereal day. The stars have not moved; more precisely, theyhave made one complete circle around the North Star and returned

Day 1Day 1Day 1Day 1Day 1

Day 2Day 2Day 2Day 2Day 2BBBBB

CCCCC

AAAAA

4 minutes4 minutes4 minutes4 minutes4 minutes

A sidereal day is theA sidereal day is theA sidereal day is theA sidereal day is theA sidereal day is thetime it takes the earth totime it takes the earth totime it takes the earth totime it takes the earth totime it takes the earth tospin once relative to the starsspin once relative to the starsspin once relative to the starsspin once relative to the starsspin once relative to the stars(point A to B). A solar day is 4(point A to B). A solar day is 4(point A to B). A solar day is 4(point A to B). A solar day is 4(point A to B). A solar day is 4minutes longer (point A to C).minutes longer (point A to C).minutes longer (point A to C).minutes longer (point A to C).minutes longer (point A to C).

SunSunSunSunSun

Earth’sEarth’sEarth’sEarth’sEarth’sorbitorbitorbitorbitorbit

75Rotation

to their original position. But notice that the sun is now about 1° to

the east of its position the day before and is not yet on the meridian.Notice also that the date has advanced by one day but the time has

decreased by 4 minutes. Now change the time step to 4 minutes and

press the Single Step Forward button to complete one solar day.The sun is now back in the same place in the sky, but the stars have

shifted slightly to the west. This four-minute period is the difference

between a solar and a sidereal day – the difference between onerotation of the earth relative to the sun and one rotation relative to

the stars. Change the time step back to sidereal days and continue

to step forward in time to make the concept clear: a sidereal day isone rotation of the earth relative to the stars, not to the sun.

Hours, Minutes, and SecondsWe divide each day into 24 hours, each hour into

60 minutes, and each minute into 60 seconds. Theseare arbitrary divisions, and we could just as well dividethe day into 10 hours of 100 minutes each, as was donefollowing the French Revolution (the French decimal daywas unpopular and did not last). Our hours come fromancient Egypt, where the day was divided into 10 “hours”of equal length with one more each for morning andevening twilight. The night was divided into 12 equaldivisions for symmetry, giving a total of 24 hours for thecomplete cycle. The custom of dividing periods of timeinto units of 60 dates to the Sumerians, who liked touse numbers that were evenly divisible by many othernumbers; 60 is evenly divisible by 30, 20, 15, 12, 6, 5, 4,3, and 2.


Time Zones

Early last century, most large American cities maintained an

observatory, a major function of which was to determine the localtime. The time in Detroit was different than the time in Toledo, and

in fact it was different in every city across the land. The custom of

using a unique local time in each community became more confus-ing as the speed of communication improved, and with the advent

of rapid rail travel it became intolerable. It was impractical to

maintain railroad timetables that had to allow for not only the speedof the train but also for local times at each station, and Canadian

railroad magnate Sir Sanford Fleming lobbied successfully for

standard time zones.In 1884 the earth’s globe was divided into 24 meridians, each

about 15° of longitude apart, starting with the zero (or prime) meri-

dian which passes through London, England. Each time zone is onehour wide. All areas within a time zone have the same solar time,

and it is one hour later in the time zone to the east. The time zone

boundary lines are not perfectly straight, but generally followpolitical and natural features for the convenience of people living

within them. This means that it is not possible to determine your

time zone from your longitude alone (as an extreme example, Chinaspans about 60° of longitude, but the entire country shares the same

time zone). Starry Night automatically adjusts the time zone if you

select a city from the list in the Viewing Location window. If yourcity is not listed and you have to enter your viewing co-ordinates

directly, you will also need to specify the time zone. You can find

time zone information from http://www.starrynight.com.Having standard time zones is a great convenience, but there

are astronomical compromises. It may be the same moment by the

clock at all locations within a time zone – but the sun and stars are

77Rotation

not at the same place in the sky for all those locations. If the sun

sets at 7:01 p.m. for a city at the eastern end of a time zone, it willstill be well above the horizon as seen from a city near the western

edge of the same time zone, where it will not set until almost 8:01

p.m. Likewise, moonrise and moonset times are not the same for alllocations within a time zone. Starry Night gives you customized

sunrise, sunset, moonrise, and moonset times for your location,

which may differ from times published in local newspapers.To see how rise and set times differ between localities, use

Starry Night to find sunrise and sunset times from your home

location. Remember that this information is contained in the sun’sInfo Window, which you can show by double-clicking on the sun.

Now change your viewing location to another city in the same time

zone [select a city by clicking the Lookup button from the ViewingLocation menu under the Go menu] and get the new rise and set

times to see the difference. Notice that moving north or south has

much less effect on the rise and set times than moving an equivalentdistance east or west.

A further twist to time zones came with the introduction of

Daylight Saving Time during World War I. Originally called “WarTime,” the purpose of moving sunset back one hour was to reduce

fuel consumption for lighting and increase production in poorly lit

factories during the late evening. It was dropped at the end of thewar, and then reintroduced during World War II. It was dropped in

1945, and reintroduced most recently in 1967. It presently begins

on the first Sunday in April and ends on the last Sunday in Octoberin most of the United States and Canada; in Europe (where it is

called Summer Time) it runs from the last Sunday in March until

the last Sunday in September. Daylight time is great for kids wholike to play outside late at night, but it delays stargazing by the

same hour.


If you have your computer clock set up correctly, Starry Night

will detect whether or not daylight time is in effect for your homelocation on the current date. The small sun icon shown to the left of

the time tells you if daylight time is on; when the icon of the sun is

bright yellow, daylight time is on, and when the icon is dimmed,daylight time is off. Just click on this icon to turn daylight time on

or off.

At any given moment – right now, for example – it is 24 differenttimes around the earth – one time for each time zone (a messy

complication is that some jurisdictions have chosen to set their clocks

using fractions of a time zone and are one-half or even one-quarterof an hour off standard time). This is a problem if you want to publish

the time of an event in a table that is useful for the entire world.

Astronomers find it effective to express events in Universal Time,abbreviated UT, which is the local time at the Greenwich

Observatory in London, England. You need only know the differ-

ence between your clock time and Greenwich time to apply correc-tions (for example, Eastern Standard Time is five hours earlier than

Greenwich). Converting local times back to Universal Time when

communicating with other observers around the world is aconvenience to people who then need only to apply the familiar

conversion of Universal Time to their own time. If you use Starry

Night to view the sky from somewhere other than the Earth’s surface,the time will automatically be shown as Universal Time.

Astronomers often need to know the interval between two times

separated by days, months, or years – for example, the exact intervalbetween two observations of a variable star made months apart.

Our calendar’s many irregularities make it laborious to find, for

example, the interval between 1:17 a.m. March 15, 1988 PST and4:06 a.m. September 12, 1999 EDT. The Julian Day system lets

you do this easily. This system assigns consecutive numbers to days

79Rotation

starting at an arbitrary “day zero” in 4713 BC. To further simplify

calculations, the Julian Day system uses decimal days rather thanhours and minutes. Convert each of the two dates to a Julian Day,

and then subtract. In the example above, the first date converts to

Julian Day number 2447235.88681 and the second to Julian Daynumber 2451433.83750, and subtract; the answer is 4197.95069

days.

To find the Julian Day equivalent of any date and time usingStarry Night, first change the time to the time you are interested in.

In Starry Night Backyard, click on any object in the Planets List

and then click the “Edit” button in the same window. This brings upthe Orbit Editor window, which displays the Julian Day in the upper

left corner. In Starry Night Pro, just choose “Guides | Onscreen

Info Settings” from the main menu and then click the “Julian Day”option.

The Nightly Rotation of the Sky

The sky turns overhead day and night, rotating as if it were a

giant crystalline sphere (which it was thought to be during Medievaltimes) with the sun, moon, planets and stars attached to it. It rotates

as a single unit at the rate of 1 r.p.d. (rotation per day).

Revolution vs. RotationTwo words that are confused endlessly are rotationrotationrotationrotationrotation

and revolutionrevolutionrevolutionrevolutionrevolution. Objects rotate on their axes and revolvearound another object. The earth rotates once a dayand revolves around the sun once a year. The word rotaaaaatehas an aaaaa for aaaaaxis, which is what an object rotates on.


As you saw in the exercise for chapter 1.2, the North Star,

Polaris, stays motionless in the sky. It is the pivot around which therest of the stars turn. Or at least it appears to be. Open the file “NCP”

to watch the rotation of the stars. Now zoom in using the Magnify

button until your field of view is about 10°. You can now see thatPolaris is not really motionless, and it makes a circle like any other

star. The true pivot point is the North Celestial Pole, which is the

point in the sky directly above Earth’s North Pole. Turn on the displayof the North Celestial Pole by selecting “Guides | Celestial Poles”

(“Guides | Equatorial | Celestial Poles” in Starry Night Pro). Polaris

is not exactly at the sky’s pole, but is 3/4° from it – pretty close!This means that it makes a circle around the North Celestial Pole,

1-1/2° in diameter, and the North Star is never more than 3/4° from

true North – too small a discrepancy to make a difference for mostnavigators. When you have finished watching the motion of Polaris,

close the file “NCP”.

Stars near the North Star make small circles around it, complet-ing one circle in 24 hours.

Stars farther from the

North Star make larger cir-cles, but still one circle per

24 hours. Stars far enough

from the North Star setbelow the northwest

horizon, disappear briefly

below the northernhorizon, and then rise

again in the northeast.

As you learned in the exercise for chapter 1.2, the far northernstars that do not set are called circumpolar (in the Southern

hemisphere, it is the far southern stars that are circumpolar). They

were considered immortal in ancient Egypt and they were associated

Long time-exposure photographLong time-exposure photographLong time-exposure photographLong time-exposure photographLong time-exposure photographcentered on the North Star.centered on the North Star.centered on the North Star.centered on the North Star.centered on the North Star.

81Rotation

with the pharaoh – who also was immortal, and who ascended to

and lived among the far northern stars once his life on earth wasfinished. Although we do not think of them as sacred today, the

circumpolar stars have the distinction of not setting and thus are

theoretically visible throughthe year (theoretically because

they may come too close to the

northern horizon to see inpractice). The Little Dipper,

Cassiopeia, Cepheus, and most

of Draco are circumpolar asseen from Canada and most of

the United States and Europe.

How a star appears to move across the sky as the sky rotatesdepends on its position on the celestial sphere. If you live at mid-

northern latitudes (anywhere within the United States and Canada

below the Arctic Circle), a star (or planet, or the sun, or moon) thatrises due east reaches its highest point when it is due south (when it

is on the meridian), and it sets due west. Stars that rise south of due

east do not reach as high a point on the meridian, and they set corres-pondingly south of due west. Stars that rise far to the south of east

make short arcs across the southern sky, are not very high when on

the meridian, and set shortly after rising in the southwest. You caneasily visualize that there is a portion of the sky that lies farther

south than the part you can see and that remains below the southern

horizon.Now we will see how a change in your latitude changes the

rotation of the sky. We will begin at a latitude of 15° N by opening

the setting “15N”. Note how low Polaris is in the sky. Find the starsVega and Eltanin on the left side of the screen. Now press the

Forward button in the time controls to start time running. Both Vega

and Eltanin fall beneath the horizon before reappearing.

The familiar W made byThe familiar W made byThe familiar W made byThe familiar W made byThe familiar W made byCassiopeia’s brightest stars isCassiopeia’s brightest stars isCassiopeia’s brightest stars isCassiopeia’s brightest stars isCassiopeia’s brightest stars iscircumpolar to most observers incircumpolar to most observers incircumpolar to most observers incircumpolar to most observers incircumpolar to most observers inthe Northern hemisphere.the Northern hemisphere.the Northern hemisphere.the Northern hemisphere.the Northern hemisphere.


Open the file “45N” to see how the sky looks from a latitude of

45° N. The North Star is farther above the horizon. Again find thestars Vega and Eltanin on the left side of the screen and start time

running. At this latitude, Eltanin is circumpolar, while Vega still

falls below the horizon briefly before reappearing.Now open the file “60N” to view the sky from a latitude of 60°

N. The North Star is still higher. Again find Vega and Eltanin and

start time running forward. From this latitude, you can see that bothVega and Eltanin stay above the horizon at all times. It is a rule that

as you move farther from the Equator, the number of stars that are

circumpolar increases. At the same time, a proportionately greaterarea of the southern sky remains invisible below the southern

horizon, and stars you might have once seen near your southern

horizon no longer rise into view.These ideas are carried to their logical extreme at the North

Pole. Open the file “90N” to view the sky from the North Pole.

From here, the entire sky is circumpolar. Start time running to seethat no stars rise and no stars set; they all remain visible all the

time, and each star maintains a constant altitude above the horizon.

The North Star cannot be seen. This is because it is directly overhead.Remember that the altitude of the North Star is equal to your latitude

(as you learned in the exercise for chapter 1.1), which is now 90°. If

you scroll up in the sky, you can see the North Pole almost exactlyat the zenith.

Scroll around to the south to find Saturn and Jupiter. If you

follow their motion, you can see that they (like all the planets, aswell as the moon and sun) also circle endlessly at a constant

elevation, neither rising or setting. In chapter 4.2, we will see how

and why these bodies do rise and set as seen from the pole; they dorise and set, but not because of the earth’s rotation.

The flip side of being at a location where all stars are

circumpolar is that many stars and constellations which are promi-

83Rotation

nent from mid-Northern latitudes cannot be seen at all from the

North Pole. Choose “Edit | Find” (“Selection | Find” in Starry NightPro) and type “Rigel”. A window will come up telling you that Rigel

is beneath the horizon. You can run time forward and wait as long

as you want, but this bright star in Orion will never come above thehorizon.

From any location, half of the sky is visible at any given

moment. However, the closer you are to the Equator, the greater thenumber of stars that rise and set, meaning that you can see more

different stars, but they are above the horizon for a shorter period of

time.Now move to the equator by opening the file “Equator”. You

can’t see the North Star although it is directly in front of you, because

it is right on the horizon. Its altitude is equal to your latitude, whichis now 0. If you start time running forward, you can guess its location

by seeing how the stars still pivot around a point. Now scroll around

so that you are facing south. The stars in this half of the sky pivotaround a point on the horizon that is due south. This is the SouthCelestial Pole. The way the sky turns in the north is symmetrical

with the way in turns in the south. Stars that rise due east pass directlyoverhead and set due west. From this location, all stars rise and set;

no stars are circumpolar. Along the Equator (and nowhere else on

Earth), every part of the celestial sphere is visible at some point intime.

Now we will move south. Open the file “30S” to view the sky

from a latitude of 30° South. A large area of the sky that includesthe Southern Cross is circumpolar. Notice that there is no bright

South Star (see the section on Crux in Chapter 5.3) and the sky

rotates around an empty, starless point (the few stars near the SouthCelestial Pole are quite faint, and can only barely be seen in perfect

conditions, making them almost impossible to use as navigational

aids). Scroll around so that you are facing north, and you will see


familiar northern

constellations inunfamiliar positions

(unless, of course, you

live in the southernhemisphere); some are

upside down and others

are on their side. Thesun still rises in the east

(the earth still rotates so

that the sky turns from east to west) – but the sun moves across thenorthern sky, not the southern as we see it from North America.

Finally, travel to the South Pole by opening the file “90S”. The

South Celestial Pole is directly overhead. All visible stars are cir-cumpolar, and each star maintains a constant elevation as the sky

turns. The sky rotates in a similar manner as from the North Pole,

but the stars you see are completely different. Not a single star inthis hemisphere of the sky can be seen from the North Pole! If you

scroll around, you will see the Southern Cross two-thirds of the

way up the sky, Sirius is 17° above the northern horizon, and Orionis upside down with his head below the horizon and his feet in

the air.

From everywhere on Earth, the sky rotates once a day, but howthe stars rise and set, which are circumpolar (and which are visible

at all), depends on your latitude.

No bright star stays near theNo bright star stays near theNo bright star stays near theNo bright star stays near theNo bright star stays near theSouth Celestial Pole.South Celestial Pole.South Celestial Pole.South Celestial Pole.South Celestial Pole.

3.2

REVOLUTION

The second major motion of the earth is its yearly revolution

around the sun. It affects how we see the sun, stars, and planets.

The Year

Just as the day is a unit of time based on the Earth’s rotation on

its axis, the year is based on the revolution of the Earth around the

Sun. The time it takes the earth to orbit the sun once – or the sun tocircle around the sky relative to the stars – is one year. In chapter

2.2 we found that there are several definitions for something so

simple as the length of the day, and it is the same with the length ofthe year. The length of the annual cycle depends on which refer-

ence point is used.

If we measure the time it takes the sun to circle the sky onceand return to the same position relative to the distant stars, 365 days

6 hours 9 minutes and 10 seconds will pass. This is the sidereal


year, sidereal meaning “stellar” or “of the stars” in Latin. However,

it is not the most useful year for most purposes because it does notallow for precession.

Precession (which is described more fully in chapter 3.3) is the

slow wobbling of the earth’s axis in a 25,600-year cycle. Precessioncauses the vernal equinox – and all other points along the ecliptic –

to shift in the same long 25,600-year cycle. If we used sidereal years

for our calendars, the seasons would slip through the months andeventually the Northern Hemisphere winter would begin in July.

As it is more convenient to keep the seasons constant – so that the

vernal equinox always occurs on or near March 21, for example –we measure the year as the length of time it takes the sun to circle

the sky relative to the vernal equinox. The vernal equinox, you will

recall, is the intersection of the celestial equator and the ecliptic.This point of intersection precesses slowly westward along the eclip-

tic, and it takes the sun less time to return to the vernal equinox than

to the same point relative to the stars. This so-called tropical year,on which our calendar is based, is 365 days 5 hours 48 minutes and

46 seconds long from vernal equinox to vernal equinox. The time it

takes the sun to circle the sky once and return to the westward-moving vernal equinox is a few minutes less than it takes the sun to

return to the same position among the stars.

Open the file “Vernal” and notice how the vernal equinox movesrelative to the stars. Use the Single Step Forward button in the time

controls to step forward through time and watch the vernal equinox

slide to the right along the ecliptic relative to the stars. It is importantto understand how slow this change is occurring. To notice it at all,

you are zoomed in to a very high magnification and advancing time

by steps of 365 days. Return to a regular 100° field of view and stepforward a few more times. At this scale the movement of the vernal

equinox with respect to the stars is imperceptible, which is why the

difference in the lengths of the two types of years is only twentyminutes.

87Revolution

Seasonal Changes In the Path of the Sun

You learned in the section in chapter 2.2 that the ecliptic is

inclined to the celestial equator. These two “great circles” (circleson a sphere) intersect at both the vernal equinox and the autumnal

equinox. The angle these two great circles make with each other at

that point is 23-1/2°, which is the amount the earth is tilted on itsaxis. You can also think of it as the angular distance between the

North Ecliptic Pole and the North Celestial Pole. The technical term

for this tilt is the obliquity of the ecliptic. If the earth sat upright inits orbit (as Mercury does), the two poles would coincide – and so

would the ecliptic and the celestial equator. This tilt is not just a

curiosity – without it, we would not have seasons!Unless you live on the equator, you know that the changing

seasons bring changes in the daily motion of the sun. The coming

of winter means that the sun rises later, sets earlier, and follows alower path across the sky. When winter fades into spring and then

The 23.5 degree tilt in the earth’s axis creates the seasons. At theThe 23.5 degree tilt in the earth’s axis creates the seasons. At theThe 23.5 degree tilt in the earth’s axis creates the seasons. At theThe 23.5 degree tilt in the earth’s axis creates the seasons. At theThe 23.5 degree tilt in the earth’s axis creates the seasons. At thesolstice on June 20, the earth’s northern hemisphere is at asolstice on June 20, the earth’s northern hemisphere is at asolstice on June 20, the earth’s northern hemisphere is at asolstice on June 20, the earth’s northern hemisphere is at asolstice on June 20, the earth’s northern hemisphere is at amaximum tilt toward the sun, while the southern hemisphere is atmaximum tilt toward the sun, while the southern hemisphere is atmaximum tilt toward the sun, while the southern hemisphere is atmaximum tilt toward the sun, while the southern hemisphere is atmaximum tilt toward the sun, while the southern hemisphere is atits furthest tilt away from the sun. The situation reverses at theits furthest tilt away from the sun. The situation reverses at theits furthest tilt away from the sun. The situation reverses at theits furthest tilt away from the sun. The situation reverses at theits furthest tilt away from the sun. The situation reverses at theother solstice on Dec. 21. The mid way point, called the Equinox,other solstice on Dec. 21. The mid way point, called the Equinox,other solstice on Dec. 21. The mid way point, called the Equinox,other solstice on Dec. 21. The mid way point, called the Equinox,other solstice on Dec. 21. The mid way point, called the Equinox,defines Spring and Autumn.defines Spring and Autumn.defines Spring and Autumn.defines Spring and Autumn.defines Spring and Autumn.

23.523.523.523.523.5°NNNNN

NNNNN

NNNNN

NNNNN

SSSSS

SSSSS

SSSSS

SSSSSWinterWinterWinterWinterWinter

SummerSummerSummerSummerSummer

WinterWinterWinterWinterWinter

SummerSummerSummerSummerSummer

SpringSpringSpringSpringSpring

SpringSpringSpringSpringSpring

AutumnAutumnAutumnAutumnAutumn

AutumnAutumnAutumnAutumnAutumn

Dec. 21Dec. 21Dec. 21Dec. 21Dec. 21

JuneJuneJuneJuneJune2020202020


into summer, these changes are reversed. The cause of these changes

is the sun’s changing position along the ecliptic.Open the file “South” to watch the sky turn as a night passes.

We are facing south with the celestial equator and ecliptic displayed,

and have chosen to view from Anchorage, Alaska, because thecelestial equator is relatively low in the sky and easy to see. Use the

Single Step Forward button in the time controls to step forward

through time by 5-minute intervals. Notice that the celestial equa-tor (and all lines of declination) remains in the same position all

night long. The celestial equator is an arc that extends from east to

west and that has a maximum elevation, in the south, that is thecomplement of your latitude (90° minus your latitude). Points on

the celestial equator move westward as the sky turns, but it is im-

portant to note that the celestial equator itself remains in the sameposition all night long. This is true no matter where you are on

earth.

In contrast, you can see that the ecliptic “wobbles” as the skyturns. In the northern hemisphere, the part of the ecliptic that runs

through Scorpius and Sagittarius is low in the south when it is in

view, while the part that runs through Taurus and Gemini is higherand much nearer to overhead when it is in view. This causes the sun

– which stays on the ecliptic – to be high in the sky in summer,

when it is in Gemini, and low in winter, when it is in Sagittarius. Ifyou keep stepping forward in time until the sun comes into view,

you will see that it is along one of the lower parts of the ecliptic,

which is not surprising, because the month is December!To see how the sun’s elevation changes during a year, open the

file “SunElevation”. The location is set for Iceland so the sun will

appear low in the sky, but after running this demonstration youshould change to your home location and run the demo again. The

sun is locked and will stay centered in your screen.

89Revolution

We begin with the sun on the meridian on June 21, which is the

summer solstice – the first day of summer. Double-click on the sunto bring up the Info Window. If you choose local co-ordinates, you

will see that the sun has an elevation (or altitude) of 49° as seen

from Iceland. This is as high as it will be that day.Now use the Single Step Forward button in the time controls to

step through one year at the rate of one solar day per step, watching

the sun run eastward along the ecliptic while remaining due southand on the meridian. The sun’s elevation decreases daily, slowly at

first – through July – but at a faster rate as summer ends. On August

1 it is 44° high at local noon. (You will see the sun deviate from themeridian slightly. This deviation is called the equation of time and

it is explained in the sidebar below) The rate at which the sun loses

altitude is greatest on the autumnal equinox, which is on or verynear September 22. This should not surprise you, as you have

probably noticed in your own experience that the length of the day

(the amount of time that the sun is above the horizon) shrinks veryrapidly with the onset of autumn. September 22 is the date when

the sun crosses the celestial equator in a southward direction, and it

marks the first day of autumn in the Northern Hemisphere. As seenfrom Iceland, the sun has a meridian elevation of 26°. This is 23°less than its elevation on the summer solstice.

Following the autumnal equinox, the sun continues to movelower at noon each day, but at a slower rate of change. Each day the

sun is a little lower at noon than the day before until, on December

21 or 22, the sun reaches its lowest “maximum daily elevation.”This is the winter solstice, which comes from Latin words for “sun-

stand” because the sun “stands still” (does not continue to lower

altitudes) before reversing itself and increasing its elevation. FromIceland, this minimum noontime elevation on the solstice is a scant

2-1/2°! The sun barely rises.


Equation of TimeThe sun is not a uniform timekeeper for two reasons.

First, the earth’s orbit is not circular, so the earth’s speedaround the sun is not constant. According to the laws ofplanetary motion discovered by Kepler almost fourcenturies ago, a planet travels fastest when it is closestto the sun (because it feels a greater pull of the sun’sgravity when it is closer to the source of the pull). Theearth’s orbit deviates from a perfect circle by less than2%, but that is enough to notice. The earth is closest tothe sun (at perihelionperihelionperihelionperihelionperihelion) early in January each year, andthen it is moving slightly faster than when at its farthestpoint (at aphelionaphelionaphelionaphelionaphelion) early in July. The earth’s changingspeed around the sun makes the sun appear to changespeed as it moves on the ecliptic around the sky.

Second, the sun travels along the ecliptic rather thanthe celestial equator. The ecliptic is inclined to the equatorbut we measure time along the equator. Even if the suntraveled at a uniform rate along the ecliptic, its projectedmotion on the equator would change during the year.

The combination of these two effects means thatthe sun is not in the same place in the sky at the sametime each day. Its altitude (height above the horizon)and azimuth (direction along the horizon) both changeover the course of a year. If we plot the sun’s position inthe sky at noon each day over the course of a year, ittraces out a figure-8 path known as the analemma. Tosee this in Starry Night, select “Go | Analemma”.

Clocks (and societies) run best at a uniform rate.Your wristwatch ignores the seasonal variations in the

91Revolution

speed of the sun and displays time as if the sun traveledat a constant speed. This imaginary uniform-speed sunis called the “mean sun” (“mean” meaning average).The difference between true (or apparent) solar timeand mean solar time is called the equation of timeequation of timeequation of timeequation of timeequation of time, andit can be anywhere between -14 and +16 minutes. Yourwatch keeps mean solar time, but a sundial keeps truesolar time.

After the winter solstice, the sun reverses its downward trendand stands a little higher at noon each day. By the spring or vernalequinox on March 21, it is back to the same noontime elevation as

on the autumn equinox: 48°. It reaches its highest noontime eleva-tion at the summer solstice in June, when it “stands still” again

before reversing course and heading southward. Follow the path of

the sun over an entire year.The changing elevation of the sun means that the relative lengths

of day and night also change with the seasons. The winter solstice

is the shortest day of the year in the Northern Hemisphere. Betweenthe winter solstice and the summer solstice, the sun’s maximum

elevation is increasing, so the sun is above the horizon longer, hence

the days become steadily longer. On the day midway between thetwo solstices - the vernal equinox- night and day are both 12 hours

in length, a fact which holds true anywhere on Earth. After the

summer solstice, the sun’s maximum elevation begins to decrease,so the days get shorter, and keep getting shorter until the Winter

Solstice comes again, becoming the same length as the nights on

the autumnal equinox. In the Southern Hemisphere, this pattern isexactly reversed.


Effects of Latitude

The maximum daily elevation of the sun changes with the

seasons from every location on earth, but the changes become morepronounced as you move farther from the equator. Two sets of

latitude lines on the earth are particularly important, and the sun’s

behavior changes when you cross one of these lines. The twoimportant sets of lines are:

1) the Tropic of Cancer and the Tropic of Capricorn

2) the Arctic and Antarctic Circles.You will learn why these lines are important in the next two

sections.

The Tropics

In our view from Iceland, the sun was never overhead. It reacheda maximum noontime elevation of 49° on June 21, but went no

higher. If you were south of Iceland, the sun would be higher on the

meridian on June 21 (and on every other date). If you were 5° southof Iceland, for example, the sun would be 5° higher in your sky at

noon on the same date. If you were just north of Mazatlán, Mexico,

at a latitude of 23-1/2°, the sun would just barely reach the zenithon the summer solstice. This latitude, called the Tropic of Cancer,

marks the northernmost limit on the earth’s surface where the sun

can be overhead. If you are north of the Tropic of Cancer, the suncan never be overhead; if you are on the tropic, the sun is overhead

only on the summer solstice.

If you are south of the Tropic of Cancer, the sun can be northof overhead. If you are a slight distance south of the tropic, the sun

is north of overhead at noon only a few days or weeks of the year.

The farther south you are, the greater amount of time the sun spendsnorth of overhead.

93Revolution

If you are on the equator, the sun spends the same amount of

time north of the zenith as south of it. Open the “Equator” setting

and step forward a day at a time by using the Single Step Forwardbutton to track the sun’s location at noon each day, as seen from the

Equator. When the Sun appears above the zenith on the screen, it is

in the northern half of the sky, and when it appears beneath thezenith, it is in the Southern half of the sky. The sun is directly over-

head at noon on the two equinoxes.

As you head south of the equator, the sun spends more timenorth of overhead than south. It is exactly overhead, however, only

twice a year.

The line of latitude that lies 23-1/2° south of the equator – theTropic of Capricorn – marks the southernmost line on the earth’s

surface where the sun passes overhead. If you are south of this tropic,

Sunshine and the Earth’s CircumferenceBy the time of the ancient Greeks it was understood

that the earth is a sphere and that its sphericity causesthe sun to sit higher in the sky as seen from latitudescloser to the equator than from northern latitudes, wherethe sun shines down at a shallower angle. In the thirdcentury BC Eratosthenes noted that the sun shonevertically down a well in Aswan, Egypt, at the summersolstice, but on the same date it was 7° south of over-head at Alexandria, 800 kilometers (500 miles) to thenorth. He assumed that the distance to the sun was sogreat that its rays struck all parts of the earth on paral-lel paths, and correctly concluded that the distance fromAlexandria to Aswan was 7/360 of the circumference ofthe earth.


the sun is always north of overhead.

The area of the earth’s surface between these two lines is called,simply, the “tropics”. These names – Tropic of Cancer and Tropic

of Capricorn – reflect ancient history. When the tropics were named,

the sun was in Cancer on the summer solstice in the NorthernHemisphere, and in Capricorn on the winter solstice. This is no

longer the case, as you will learn in chapter 3.3.

Arctic and Antarctic Circles

The tropics mark the limits on the earth’s surface where thesun can be overhead. Two other lines on the earth’s surface mark

the extreme points where the sun does not set – or does not rise.

These are the Arctic and Antarctic Circles.Move to the Tropic of Cancer by opening the file

“WinterTropic”. As with all points north of the Equator, the sun is

at its lowest noontime elevation on the winter solstice. Double-clickon the sun to bring up the Info Window, and read the sun’s elevation:

43°. This is 20° higher than the sun’s maximum elevation as seen

from Chicago on the same date. From the tropics, the sun is rela-tively high even at the winter solstice (which is why it is not cold in

the tropics in winter). The farther north you are, the lower the sun is

at noon.As we move northward along the Earth’s surface, the maxi-

mum elevation of the Sun at midday on the winter solstice decreases.

For every degree of latitude we move north, the maximum elevationof the sun is one degree lower. At some latitude the sun’s elevation

will be zero; on the winter solstice the sun will just barely rise before

setting immediately again. This line of latitude - called the ArcticCircle – lies 23-1/2° south of the North Pole at a latitude of 66-1/2°.

If you move north of the Arctic Circle, the sun does not rise at all on

95Revolution

the winter solstice. The farther north you are of the Arctic Circle,

the greater the number of days in the year when the sun does notrise. At the pole itself, the sun does not rise for six months!

Let’s position ourselves at the Arctic Circle just before noon

on the winter solstice by opening the file “ArcticWinter”. Stepthrough time at 10-minute intervals using the Single Step Forward

button. The centre of the sun barely touches the southern horizon at

noon before setting again.Now open the file “ArcticSummer”. You are viewing from the

same location on June 21, the summer solstice. The sun is much

higher in the sky – it has a noon-time elevation of 47°. The sun’srays do not warm the ground very effectively at such an angle, so it

is safe to predict a cool June day for the Arctic Circle. Step forward

through the rest of the day until sunset – and discover that thereisn’t one! The sun barely touches the northern horizon at midnight,

but it does not set. You are in the “land of the midnight sun” – at

least on this one day, the summer solstice.If you have ever traveled far north – to Alaska, Scotland, or

Norway – in the summer, you’ve noticed that it gets dark very late

at night or not at all. It is amazing to be able to read a newspaper at11 p.m. However, if you’ve been at the same place in mid-winter

you’ve been equally amazed that it remains dark until late morning

and is not terribly bright even at noon; people often have to uselights in the house all day long in winter.

The Arctic Circle is the southernmost latitude that experiences

a midnight sun, and then only for one day – the summer solstice. Asyou move north of the Arctic Circle, there are more days surround-

ing the solstice when the sun does not set. At the North Pole, the

sun rises on March 21 (the vernal equinox) and sets on September21 (the autumnal equinox) and there are six months of midnight

sun. That is followed by six months of no sun (but not of no light –


it takes a long time for the sky to get dark once the sun has set at the

north pole.)

There is a corresponding latitude 23-1/2° north of the SouthPole called the Antarctic Circle where all of this is true too, but six

months out of phase. When the north polar regions are experiencing

continuous sunlight, the south polar regions are in continuousdarkness. When the sun rises at the North Pole, it sets at the South

Pole. When thinking about seasons and daylight, the earth is very

symmetrical about its vertical axis.

Daylight QuizTest your understanding. Which latitude has more

hours of total sunlight total sunlight total sunlight total sunlight total sunlight during the year – the equator orpole? Think about it – you have enough information tofigure it out.

Answer: None of the above. Ignoring clouds, all partsof the earth’s surface receive the same number of hoursof sunlight during a year, which is a total of exactly sixmonths worth. At the equator, the sunlight comes in 12-hour batches every day with little variance from one dayto the next through the seasons; at the pole, it comessix months at a time, all at once, from March throughSeptember. Why, then, is it colder at the poles? Becausethe sun’s rays always strike the poles at a shallow angle,while at the equator the sun shines down fiercely fromnearly overhead 12 months of the year.

97Revolution

The Sun Also Rises … But Where?

So far we have been thinking about the sun’s height above the

horizon at noon and how it changes during the year. You couldmonitor the sun’s height to track the seasons, and if you know the

sun’s maximum altitude, you know the date. Let us now think about

seasons in terms of the sun’s rising and setting points on the horizon.Open the file “SunElevation2”, and we are back in Iceland on

the vernal equinox. It is 7:30 a.m. and the sun has just risen. Notice

where on the horizon the sun rose; it rose due east, or 90° azimuth.Step slowly step through the calendar at one-day intervals for a

few weeks, and stop. You will notice two things happening. Each

day the sun is north of and higher than its position the day before.It is higher each succeeding day at the same time (e.g., 7:30) because

it rises earlier, and we have already discussed why this is so; now

we want to keep track of the position on the horizon where it rises.Ignore the sun’s changing altitude, and resume running forward

through time. The sun’s rising point moves northward through the

months at a decelerating rate until it reaches its northernmost risingpoint; it pauses its northward travel before beginning to move south

again. Note the date – it should be the summer solstice, June 21.

Bring up the Info Window to see that the sun rose at 3:02 a.m.Change the time to 3:02 a.m. to put the sun on the horizon, and find

its azimuth by bringing up the Info Window again. On June 21, as

seen from Iceland, the sun rises in the northeast at an azimuth of21°.

The Info Window also shows that sunset will occur at 11:56

p.m. Change the time to this time and find the sun again on screen,setting in the northwest. Bring up its Info Window again. If you

check its azimuth, you will find that it is just under 339°, or 21°west of north (360 - 21 = 339). The sun’s setting position each day


is always symmetrical with its rising position. If the sun rises 30°north of due east one day, it sets 30° north of due west that sameday.

At the summer solstice the sun’s rising point is as far north as it

will ever get. On the following day, it begins to move south – slowlyat first but at an increasing rate until the rate of change is greatest at

the autumn equinox. It rises due east at the autumn equinox – as it

does on the vernal equinox. For the next six months, the sun rises inthe southeast and sets in the southwest. Continue stepping forward

in time to see this change. You may want to occasionally set the

time ahead as you step through the days in order to keep the sunnear the horizon. Notice that by late November the sun’s rising point

changes little from day to day, and the sun again “stands still” on

the winter solstice, when it reaches its southernmost rising point.From Iceland this point has an azimuth of 156°. This is as far south

of due east (66°) as the sun rose north of east on the summer sol-

stice. As the winter solstice marks the sun’s lowest noontime eleva-tion, so it marks the sun’s southernmost rising and setting points.

Perform the same experiment for your home town and find the

sun’s northernmost and southernmost rising and setting points.Because the sun rises at the same azimuth on the same date each

year, if you know the sun’s rising point on the horizon, you know

the date. (In practice, the sun rises at the same azimuth twice duringthe year – except at the two solstices – but a clever person can keep

the two straight.) Earlier cultures knew this too, and many kept

track of the date by watching the point on the horizon where the sunrose or set. In Russia earlier this century one person in each village

was appointed to sit in a certain place and monitor the sunsets to

know when the festival days should be celebrated, and some nativeAmericans still use the technique.

Finally, look at the sun’s changing and rising points from the

Arctic Circle and then the North Pole and South Pole. Ask yourself

99Revolution

what you expect to see before doing the experiments. By now, you

should have a solid understanding of how the Earth’s revolutionaround the Sun and its tilt combine to cause changes in the sun’s

motion throughout the year as seen from all parts of the earth. A

tool like Starry Night is invaluable for such visualizations.

Seasonal Changes in the Stars and Planets

While the changes in the motion of the sun are the most dra-

matic effects of the earth’s revolution, our ability to observe the

stars and planets is affected as well. This is due to the sun’s apparentmotion through the celestial sphere along the ecliptic.

We saw in the exercise for chapter 1.2 that constellations are

sometimes classified as “winter” or “summer” constellationsdepending on the season when they are best observed. The general

rule of thumb is that the farther a constellation is from the sun, the

longer it is visible at night. Sowhen is the best time to see

Scorpius? Not during the

month of November, when thesun passes through it, but six

months later, when the sun is

on the other side of the sky.This rule of thumb works best

for constellations near the

celestial equator, for theirdistance from the sun varies the

most with the changing

seasons. The farther a star isfrom the celestial equator, the

more difficult it is to group it

The Scorpion is a prominentThe Scorpion is a prominentThe Scorpion is a prominentThe Scorpion is a prominentThe Scorpion is a prominentfigure during evenings in the latefigure during evenings in the latefigure during evenings in the latefigure during evenings in the latefigure during evenings in the latespring and summerspring and summerspring and summerspring and summerspring and summer


with a certain observing season.

The motion of the moon and planets is more complex becausewe have to consider the apparent change in their position due to the

earth’s revolution, as well as the true motion due to their orbit around

the sun (or the earth, in the case of our moon). The motion of themoon is studied in chapter 4.1, and the motion of the planets is

considered in chapter 4.2.

3.3

PRECESSION

So far we’ve looked at two motions of the earth which change

the appearance of the sky: the earth’s daily rotation on its axis andits annual circuit around the sun. The third motion of the earth which

changes the appearance of the sky, and which has been mentioned

several times, is precession, also called “precession of theequinoxes.” It is the wobbling of the earth on its axis.

Precession was discovered in the second century BC by the

Greek astronomer Hipparchus. He noticed a “new star,” which wewould call a nova or supernova (an exploding star), and wondered

about the permanence of the stars. Although it was not customary

for astronomers to make actual observations of the sky (such workwas considered labor, and manual labor was assigned to slaves),

Hipparchus decided to make an accurate record of the stars, their

brightnesses, and positions. He compiled the first complete star chart– one that was used for centuries after. Like all star catalogs of the

time, it used the ecliptic co-ordinate system. In the process, he

compared stars he observed with observations recorded 150 years


previous by earlier astronomers, and he noticed a systematic shift

in the ecliptic longitudes – but not latitudes – of the stars that was

greater than could be accounted for by errors of measurement. Heconcluded that the co-ordinate system itself was shifting, although

he could have no idea why.

Since the equinoxes – the intersections of the celestial equatorand ecliptic – were shifting, and the vernal equinox marked the

zero point of the ecliptic co-ordinate system, he called the motion

the “precession of the equinoxes.” His value for the annual

Worshipping the Secret of PrecessionA mysterious religion, now extinct, once incorporated

secret knowledge of the precession of the equinoxes.This knowledge was kept so private that only in the lastfew decades was it rediscovered. Yet at one time itsfollowers were spread through the Roman Empire fromEngland to Palestine and their religion was a rival toyoung Christianity.

Worshippers of Mithras portrayed their hero slayinga bull in the presence of figures of the zodiac. Taurus,the celestial bull, died in the sense that precession hadmoved the location of the vernal equinox from Taurusinto Aries, ending the “Age of Taurus.” A force that couldmove the equinox was stronger than any other yet known,for it moved the entire cosmos. Such a force must comefrom beyond the cosmos, and it was worshipped by thefollowers of Mithraism – who kept this knowledge asecret. They certainly would have been shocked to learnthat the force originates with the pull of the sun andmoon on the earth’s equatorial bulge!

103Precession

precessional shift was within 10% of the correct value. In addition

to discovering precession and compiling the first star catalog,Hipparchus devised the stellar magnitude system that is still used

today, and was the first to specify positions on the earth’s surface

by longitude and latitude. He is justly remembered as the greatestastronomer of antiquity.

Today we know that the earth’s axis wobbles and the equinoxes

precess largely because of the gravitational influence of the moon.The earth’s relatively rapid 24-hour spin causes the earth to bulge

at its equator. The earth’s equatorial diameter is 21 kilometers (13

miles) greater than its polar diameter. The moon – and to a lesserextent the sun and to a far lesser extent the planets – pull on this

slight bulge. The bulge is oriented along the earth’s equator, but the

moon and sun pull from a different direction – from the ecliptic.The effect is to try to pull the earth into a more upright orientation.

But the earth is spinning like a gyroscope, and it resists being pulled

over. Instead, it precesses, orwobbles; the amount of tilt (in

this case, 23-1/2°) remains

constant while the direction of tiltchanges. The earth’s axis traces a

huge circle in the sky with a

radius of 23-1/2° in a time spanof 25,800 years (see illustration,

right). That is a long time – there

are as many human lifetimes inone wobble as there are days in

one year. Starry Night can show

a range of time from 4713 BC to10 000 AD, a period of 14, 700

years, or just over half of a

precession cycle. The celestial

The Earth's axis rotatesThe Earth's axis rotatesThe Earth's axis rotatesThe Earth's axis rotatesThe Earth's axis rotates(precesses) just as a spinning(precesses) just as a spinning(precesses) just as a spinning(precesses) just as a spinning(precesses) just as a spinningtop does. The period oftop does. The period oftop does. The period oftop does. The period oftop does. The period ofprecession is 25,800 years.precession is 25,800 years.precession is 25,800 years.precession is 25,800 years.precession is 25,800 years.


equator, which is always 90° from the poles, wobbles at the same

time and at the same rate as the poles.The earth’s axis meets the sky at the North Celestial Pole (abbre-

viated NCP), and as the axis precesses, the NCP changes. Right

now, the NCP is less than 1° from the North Star, Polaris, and acentury from now the NCP will be slightly closer to Polaris. Then

the NCP will move on.

Five centuries ago, when Columbus sailed the ocean blue,Polaris was 3-1/2° from the NCP, and sailors had to make allowan-

ces for this offset when navigating. At the time of the birth of Christ,

Polaris was 12° from the NCP and not a useful pole star at all. Almostthirty centuries earlier, Thuban – a faint star in Draco – was the pole

Thuban and the Great PyramidThe North Star at the time of the construction of the

Great Pyramid was Thuban, an unassuming 4th-magnitudestar in Draco the Dragon. You can find Thuban midwaybetween Mizar in the Big Dipper and the end of thebowl of the Little Dipper. In 2700 B.C the earth’s axispointed near Thuban, and the star held specialsignificance to the Egyptians, who associated it, and the“undying” stars that were circumpolar and that neverset, with the Pharaoh. The northernmost “air shaft”leading upward from the King’s Chamber in the GreatPyramid pointed to Thuban, symbolically connecting thedead pharaoh with the central undying star.

Thuban is corrupted Arabic for “serpent’s head.”Open the file “Thuban” to see how close Thuban was tothe sky’s pole when the pyramids were built, in2560 B.C.

105Precession

star. Through the millennia, the earth has had several pole stars –

but most of the time there has been no bright star near the NCP.Open the file “Precession”. We are in the year 1 AD, and

Augustus is the first Emperor of Rome. Notice that the grid lines

converge on the North Celestial Pole, which is near the Little Dipper.As you should know by now, Polaris is the bright star in the handle

of the Little Dipper, and is to the bottom left of the pole. The North

Celestial Pole is in the faint constellation Camelopardalis. Now stepforward through time at 40-year intervals and watch the pole move

towards Polaris. Pause near the year 1500 to notice that, to

Columbus, the North Star was about three degrees from the sky’spole (click on Polaris and drag the cursor to the North Celestial

Pole to read the angular separation in degrees). Continue moving

forward in time until you have reached the present.At present, the North Celestial Pole is less than one degree

from the star Polaris. Reduce the time step to 5 years and zoom in

until your field of view is about 10°. Continue stepping forward intime to discover that the Pole will

be closest to Polaris (about 1/2°apart) around the year 2100 beforemoving again. Restore your field

of view to 100° and change the

timestep to 100 years. Stepforward through future millennia

to discover future pole stars.

Starry Night allows you to go asfar forward as 10 000 AD. There

will not be another pole star until

about 4,000 AD when Er Rai(Gamma Cephei) will be fairly

close. In 7,600 AD the star

Alderamin (Alpha Cephei) will be

The three best north stars ofThe three best north stars ofThe three best north stars ofThe three best north stars ofThe three best north stars ofthe next 10 000 years. Thisthe next 10 000 years. Thisthe next 10 000 years. Thisthe next 10 000 years. Thisthe next 10 000 years. Thisview is from the year 4100 AD.view is from the year 4100 AD.view is from the year 4100 AD.view is from the year 4100 AD.view is from the year 4100 AD.


the north star.

Our present pole star is the best the earth will ever have. Noother star is ever as bright and as close to the celestial pole as ours

will be for the next two hundred years. It’s one of those little things

we just take for granted.Open the file “PrecessionSouth”. We are at the earth’s South

Pole at approximately the present date. The South Celestial Pole is

overhead. Notice that it is in the constellation Octans, but it is notnear any bright star and there is no “South Star”. Step forward

through time to discover future South Pole Stars. You will see that

four different stars will be excellent pole stars far in the future,including two (Aspidiske and Delta Velorum) which are comparable

in brightness to Polaris.

The sky’s pole shifts against the stars due to precession, but sodoes every point on the sky. The entire equatorial co-ordinate grid

precesses with respect to the stars, as Hipparchus discovered. The

RA and Dec of a star are valid numbers only for a particular moment,called the epoch and the co-ordinates change with time. For example,

the co-ordinates of Vega in 2000 AD are RA 18h 37m, Dec +38°47', but in 2500 AD they will be RA 18h 54m, Dec +39° 22’. Wemust specify the epoch of the co-ordinates if we are dealing with

long spans of time or extremely precise co-ordinates. Astronomers

often use J2000 co-ordinates, which are the co-ordinates an objecthad in January 2000. Now you can explain why two different

equatorial co-ordinate systems show up in an object’s Info Window

in Starry Night. One system gives the co-ordinates for the currentdate (“current” meaning whatever date you are looking at with Starry

Night), the other gives the co-ordinates for January 2000. If you

compare the two systems, the co-ordinates will be almost identical.Precession only has a major effect over a very long period of time.

107Precession

Precession & Astrology

Precession has an interesting effect on astrology, and especially

on birth signs or astrological signs. The signs – such as “Scorpio”– are each a uniform section of the sky 30° wide. They are measured

eastward along the ecliptic from the vernal equinox, which is the

intersection of the ecliptic and the celestial equator and is the zeropoint. When this system was set up around 600 BC, the zero point

was in Aries and was called the “first point of Aries.” The

constellation Aries encompassed the first 30° of the ecliptic; from30° to 60° was Taurus; from 60° to 90° was Gemini, and so on. This

scheme ignored actual stars, but uniformity was more important

than fussing about star positions.Since then, precession has caused the celestial equator to

wobble so as to cause the intersection point between it and the ecliptic

to move westward along the ecliptic by 36°, or almost exactly 1/10of the way around.

Open the file “FirstSign” and notice that the date is 600 BC.

The intersection of the ecliptic and celestial equator is in westernAries. Step forward through time in intervals of 50 years and notice

that the intersection moves westward. The ecliptic does not move,

but the celestial equator does. The vernal equinox crossed from Ariesinto Pisces around the time of the birth of Christ (and the birth of

Hipparchus), according to modern constellation boundaries. Stop

in the year 2000 and notice that the intersection, which is still calledthe “first point of Aries,” has slipped most of the way through Pisces.

In six more centuries it will leave Pisces.

Your birth sign in your morning newspaper horoscope ignoresprecession. What your horoscope calls “Aries” is the 30° segment

along the ecliptic that is east of the current location of the vernal

equinox – but most of it is in Pisces! The next 30° segment, called“Taurus” in horoscopes, is largely in Aries. In the last 2,600 years


the signs have slipped 2,600/25,800 of the way around the sky in a

westward direction, relative to the stars beyond. The astrologicalsigns are directions in space that do not correspond to the stars in

the astronomical constellation with the same name.

Precession causes the position of the sun on the vernal equinoxto shift as the earth wobbles – but so does the position of the sun on

every date. This means that it is not only the names of the Zodiac

signs that are now inaccurate. The names of the tropics are nowinaccurate as well.

Open the setting “Tropic”, which shows the position of the Sun

as seen from a viewing location along the Tropic of Cancer at thetime of the summer solstice in the Northern Hemisphere, June 21.

If you double-click on the Sun to bring up its Info Window, you

will see that the sun has an altitude of almost 90°, meaning it isdirectly overhead. But the Sun is not in the constellation Cancer!

Use the Magnify button to zoom in and see that it is actually just

inside the boundary of Taurus (it was in Gemini until only a fewyears ago). Similarly, on the Southern Hemisphere summer solstice

on December 21, the sun is in Sagittarius, not Capricornus. Why is

there a discrepancy? To find out, open the setting “OldTropic”. Thisis another view of the summer solstice as seen from along the Tropic

of Cancer, but it is

the year 150 BC,about the time that

the tropics were

named. You willsee that the sun was

in Cancer on the

Summer Solsticethen. Likewise, it

was in Capricornus

on the Winter

The winter solstice has moved far fromThe winter solstice has moved far fromThe winter solstice has moved far fromThe winter solstice has moved far fromThe winter solstice has moved far fromCapricornus in the 2600 years since theCapricornus in the 2600 years since theCapricornus in the 2600 years since theCapricornus in the 2600 years since theCapricornus in the 2600 years since theTropic of Capricorn got its name.Tropic of Capricorn got its name.Tropic of Capricorn got its name.Tropic of Capricorn got its name.Tropic of Capricorn got its name.

109Precession

Solstice (Summer Solstice in the Southern Hemisphere), as you can

also demonstrate with Starry Night, if you wish. Since 150 BC,precession has shifted the summer solstice westward from Cancer

to Gemini to Taurus and the winter solstice from Capricornus to

Sagittarius. Therefore, the Tropic of Cancer should be renamed theTropic of Taurus, and the Tropic of Capricorn should now be named

the Tropic of Sagittarius. But these names too will be ephemeral,

and the tropics will continue to shift. In another 23,200 years thecycle will have been completed and the sun will once again appear

in Cancer and Capricornus on the solstices.

This table lists the dates when the sun is actually within theastronomical constellations of the zodiac, according to modern

constellation boundaries and corrected for precession (these dates

vary by up to one day from year to year). You will probably findthat you have a new birth sign. If you were born between November

29 and December 17, you are an Ophiuchus!

Constellation DatesCapricornus January 20 to February 16

Aquarius February 16 to March 11

Pisces March 11 to April 18Aries April 18 to May 13

Taurus May 13 to June 21

Gemini June 21 to July 20Cancer July 20 to August 10

Leo August 10 to September 16

Virgo September 16 to October 30Libra October 30 to November 23

Scorpius November 23 to November 29

Ophiuchus November 29 to December 17Sagittarius December 17 to January 20


To confirm that these dates are correct, open the file

“RealZodiac”. Step forward through a year and notice the dates onwhich the Sun moves into a new constellation.

The Dawning of the Age of AquariusAccording the popular ‘60s song, “it is the dawning

of the Age of Aquarius.” What does this mean, and whendoes that happy day arrive?

The name of an astrological “age” comes from theconstellation the vernal equinox is in. During classicalGreek times, the vernal equinox was in Aries and it wasthe “Age of Aries.” By about the time of the birth ofChrist, the equinox had precessed westward until it stoodin Pisces, and the last 2,000 years has been the “Age ofPisces.” When the equinox moves into Aquarius, the“Age of Aquarius” will begin. Using modern constellationboundaries, the equinox has 9° farther to precess beforeit enters Aquarius, and that won’t happen until the year2597. Apparently we have a bit of a wait before thedawning of the age of universal peace and love. Openthe file “First Sign” again, reduce the timestep, and stepthrough time to confirm the year when the Age of Aquariusdawns. When will the Age of Capricornus begin? The Ageof Sagittarius?

111Precession

The Extended Constellations of the Zodiac

Using modern constellation boundaries, the sun travels through

the traditional 12 constellations of the zodiac plus Ophiuchus. Themoon and planets orbit on paths that are inclined to the ecliptic, and

they travel through additional constellations. Not counting Pluto,

whose orbit is inclined a whopping 17° and which would add severalmore constellations to the list, these are the 21 astronomical

constellations that are visited by at least one of the solar system’s

major bodies.

Aquarius Corvus Libra Sagittarius Virgo

Aries Crater Ophiuchus ScorpiusCancer Gemini Orion Scutum

Capricornus Hydra Pegasus Sextans

Cetus Leo Pisces Taurus

Open the file “ExtendedZodiac” and step through time to follow

the motion of the moon. You can see that it oscillates around theline of the ecliptic. Imagine making a band which is twice the width

of the maximum distance from the moon to the line of the ecliptic.

If you wrap this band around the ecliptic line, every constellationpartially or fully covered by this band is touched at some point by

the moon. The naked eye planets that travel farthest from the ecliptic

are Venus and Mars, so they make the biggest “bands” and travelthrough the most constellations.


Section 4

Our Solar System

PEOPLE who live in brightly-lit cities cannot

experience the awesome beauty of a dark night

sky with its “millions” of stars and the Milky Way

unless they travel away from home – but they can see

the moon and planets. Tracking the motions of the

moon and planets is perhaps the oldest astronomical

activity and it is one that city-folk can still fully parti-

cipate in. It is a good way to remain connected to the

sky. Starry Night excels at showing you how the moon

and planets move and where they will be in the future,

and it lets you plan your skywatching and understand

what is going on. Chapter 4.1 will look at the motions

of the moon, while Chapter 4.2 will cover the motions

of the planets. It is important to remember that the

apparent motion of a moon or planet as seen from

Earth is a combination of two things:

1) the motions of the earth (as discussed in the last

three chapters)

2) the real motion of the moon or planet around its

parent body

4.1

THE MOON

As long as people have been looking up, they have been

fascinated by both the motions and the changing appearance of themoon. Scholars debate how much was known when and to what

use monuments such as Stonehenge and others were put, but all

agree that the moon was monitored carefully and even worshippedfrom remote prehistoric times. We don’t worship it today, but we

can still follow its motions.

The moon revolves about the earth approximately 12 times peryear or approximately once every 30 days. This relatively rapid

rate makes the moon’s motion among the stars easily visible from

night to night.Because the solar system is relatively flat, the moon and planets

stay near the sun’s path along the ecliptic. The moon’s orbit is tilted

5° to the ecliptic, and so the moon remains within 5° of the ecliptic.Half the month it is north of the ecliptic and half the month south,

and it crosses the ecliptic at two points called the nodes.

Of course, Starry Night will show you precisely where the moon

Section 4 – Our Solar System116

is tonight or any other night (or day), and not only from your

backyard but from any place on earth. It is in almost the same position

in the sky for everyone on earth at the same moment, but not exactly.Observers separated by a large distance will see the moon in a slightly

different position relative to the background stars. This is an example

of parallax – the apparent shift of a comparatively nearby objectseen against distant objects when viewed from different positions.

Demonstrate by using Starry Night to view the moon from two

different cities at the same moment. Open the file “MoonParNY”file, and notice Saturn just above the moon. The time is 7:30 p.m.

on November 11, 1997, and you are viewing from New York City.

Leave this window open and now open “MoonParDetroit”, whichshows the moon and Saturn as seen at the same moment from Detroit.

Align the two Starry Night Windows so that you can see both views

at the same time. Notice how Saturn – and the background stars —are in slightly different positions relative to the moon, depending

on which city you are viewing from. In reality, it is the moon whose

position has shifted relative to the far more distant planet and stars.Moving to the far side of the earth can cause the moon to shift its

position by an amount greater than its own diameter! This is

especially important in a solar eclipse or the occultation of a planet

The Oldest Moon ObservationA series of notches cut into a reindeer bone found

buried in a cave in France may be a symbolic tally of thenightly appearances of the moon over a two-month span.This bone is 30,000 years old, suggesting that as earlyas the Ice Age people kept track of the position of themoon and counted the passing of time in months.

117The Moon

or star by the moon – the moon may align with the sun or occult a

planet as seen from one city, but not from another.

New & Full Moon

The moon’s monthly orbital cycle begins at new moon, when

the moon is in line with the sun. In reality the moon is usually a bit

above or below the sun then and not directly in front of it, so newmoon is defined as the moment when the sun and the moon have

the same longitude, which is also approximately when they have

the smallest angular separation. After one week, the moon has moveda quarter of the way around the earth and is at its first-quarter

position. One week later finds the moon opposite the sun (or at

least at longitude of the sun plus 180°), and this is the full moon.One week later the moon is three-quarters of the way around its

orbit, and we call it a third-quarter moon or last-quarter moon. After

another week it has completed its cycle and is new again.

How Long is the Moon Full?The moon is full when it is opposite the sun (when

its longitude is 180° greater than the sun’s). It is full foronly an instant, just as it is midnight (or 3:00 p.m.) foronly an instant. That instant can be calculated to thenearest second, and it is the same momentsimultaneously all over the earth. To most people,however, the moon looks full for three days to a week.For practical purposes, the moon is considered to befull for the entire night closest to the instant of itsopposition.


The Month

The time it takes the moon to complete one orbital cycle is a

month, but how long is that? By now, you are probably not surprisedto learn that there are two common definitions of the month. Open

the file “MoonMonth” to find out why.

Begin with the new moon on April 12, 2002. The time is 9:22a.m. – the actual moment when the moon is new – and we are viewing

from Honolulu, Hawaii. The moon and sun are very close together.

The stars are turned on and daylight colors are turned off. Noticethat the moon is just a fraction of a degree to the right of the star

HIP 6751. Now step forward through time at one-day intervals until

the moon approaches HIP 6751 again. This will occur on May 9,which is a period of 27 days. The precise orbital period is 27.32166

days –the time the moon takes to circle the earth once relative to the

stars, and it is called a sidereal month.But notice that the moon is not yet new! It is a thin waning

crescent. During the 27-1/3 days it took the moon to return to the

same part of the sky, the sun has moved on 27° eastward. Recallthat the sun moves eastward along the ecliptic because the earth

moves around the sun, and that the sun moves eastward with respect

to the stars at the rate of very nearly 1° per day. In order to completeits cycle of phases and become a new moon again, the moon has to

move that additional 27-1/3°, and that takes it 2-1/6 days. Step

forward in time to see that the moon and the sun come closesttogether (and the moon is completely dark) on May 12. This cycle

of the moon’s phases – from new moon to new moon – has a period

of 29.53059 days and it is called a synodic month. It is what wenormally think of as a month.

The month is a fundamental cycle in the sky, and since

prehistoric times it has formed the basis of a calendar. Early calendarswere lunar – they were based on actual observations, and a new

119The Moon

month began with the sighting of the new moon. This is still true in

Islamic culture. A problem is that there are not a whole number oflunar months in a year – there are 12-1/3 synodic months in a year.

A calendar based on synodic months has to add leap months

periodically if it is to keep the months in phase with the year. TheIslamic calendar does not have leap months, so its months (and

holy days) do not always fall in the same season. Our modern

Gregorian calendar is solar, and months are given arbitrary lengthsso they add up to 12 months of 365 days total.

The moon’s cycle also forms the basis of our week, which is

one quarter of a synodic month (rounded off to the nearest wholeday). The idea of the 7-day week originated in Babylon and was

spread around the Mediterranean world by the Jews after they were

released from Babylonian captivity.

Motion of the Moon

Because the moon stays near the sun’s path, if you know the

movements of the sun you already know the approximate move-

ments of the moon. There are differences, however. The new moonrises and sets with the sun and follows its path across the sky, but at

its other phases the moon travels a different path than the sun. Let’s

look at a new and then a full moon to see why this is so. Open thefile “MoonPath”; the time is 7:00 a.m. on October 27, 2000, and

the sun and new moon are close together and just above the

southeastern horizon. Daylight is turned off so we can actually seeboth of them. Notice where the sun and moon are on the horizon as

they rise. Step through time in 10-minute intervals and watch them

move together across the sky until they set. Note the direction inwhich they set.

Now advance time by two weeks to setting the date and time to

5:30 p.m on November 11. The moon is now full, and it is again


just above the eastern horizon. Notice how far north it is of its rising

position two weeks earlier. Advance through time at 10-minuteintervals and watch it move across the sky, and confirm that it sets

far to the north of its setting position two weeks earlier. The full

moon is 180° opposite the sun and does not follow the sun’s pathfor that day – it follows the path the sun will have in six months,

when the sun is 180° in advance of its present position. The path of

the full moon is six months out of phase with the path of the sun;the full moon in winter travels the path of the sun in summer, and

vice versa. The sun is highest in the sky (in the Northern Hemisphere)

at the summer solstice in June, but the full moon in June travels onits lowest path for the year. The full moons in December and January

pass the highest overhead at midnight. Close the file “MoonPath”.

Midnight MoonIf you are north of the Arctic Circle, the sun can

remain above the horizon for more than 24 hours andyou can see a “midnight sun” (see chapter 3.2). Themoon too can remain above the horizon for more than24 hours in a row, and at other times it does not rise atall!

Open the file “MoonArctic1” to view the “midnightmoon” from northern Greenland. Step forward thoughtime in 1-hour steps to watch it circle the horizon; atwhat time will it be due north? … when will it finally set?Because the moon traces the sun’s yearly path in only amonth, the amount of time it spends above the horizonvaries much more rapidly than does that of the sun.Close the file “MoonArctic1”.

121The Moon

Surface of the Moon

Through even a small telescope you will see more features on

the moon than on the sun, all the planets, and all the comets anddeep space objects put together. It is beyond the scope of this book

to interpret the moon’s features (consult a popular observing guide

to the sky, or visit www.livesky.com ), but let it be said that themoon’s surface is the result of impacts from above and lava flow

from below. During its first half-billion years, the moon was struck

again and again by asteroids – debris left over from the formationof the planets. They exploded on impact as their kinetic energy was

changed into heat, and they vaporized themselves and a great deal

of the rock that they hit, blasting out huge craters. By four billionyears ago the moon’s surface was so saturated with craters that not

one square centimeter hadn’t been hit at least once. Over the next

billion years, immense amounts of hot lava flooded the low areaswith smooth lava flows that cooled and turned dark. These colossal

lava flows formed what we call the lunar “seas” – seas of cooled

lava. On earth, erosion from wind and rain has erased the evidenceof the earth’s violent early history; but the moon has no atmosphere

and therefore no erosion. The lava stopped flowing long ago and

few asteroids fall today, so the moonlooks much like it did three billion

years ago.

It is a very common mis-conception that the moon has a fixed

dark side and a fixed light side. The

sun rises and sets on the moon’ssurface as it does on the earth, but

with a “day” that is 27-1/3 earth-days

long. Note however that the moondoes keep one side permanently

Sunrise as seen from theSunrise as seen from theSunrise as seen from theSunrise as seen from theSunrise as seen from themoon, with a “new earth”moon, with a “new earth”moon, with a “new earth”moon, with a “new earth”moon, with a “new earth”just to the sun’s right.just to the sun’s right.just to the sun’s right.just to the sun’s right.just to the sun’s right.


The Earth from the MoonEveryone has seen the moon from the surface of

the earth, but only 12 men — the Apollo astronauts —have seen the earth from the surface of the moon. Youcan join them with Starry Night.

Open the file “MoonApollo11”. You are at the Apollo11 landing site along the moon’s equator on July 20,1969, the day of the moon landing. The earth is highabove the western horizon, in the constellation Pisces.Double-click on the earth to bring up an Info Window tosee that its phase is 68%; it is a gibbous earth.

You cannot see the moon half the time from yourhome on earth, but there is no need to wonder if youwill see the earth from the moon. If you are on the sideof the moon that faces the earth, the earth is always inthe sky, day and night, and it never rises nor sets. Itchanges position little from day to day and year to year,but it does change its phase. The earth goes through afull set of phases once a month just as the moon does,but the two have opposite phases and remain 180° outof synchronization. When the moon is new, the earth isfull, and vice versa. Think about it a minute, and thenstep forward through time with Starry Night. You willsee the earth rotate and the stars and constellationsmove behind it, but the earth hardly changes its positionin the sky. If you wish to spend more time exploring thisconcept, open multiple windows to view from the earththe moon is new (as seen from earth), for example, it isin the direction of the sun; at that same moment theearth (as seen from the moon) must be in the opposite

123The Moon

turned toward the earth while the back side (or far side) of the moon

is permanently turned away from earth (i.e., its rotation and

revolution periods are the same). The moon is in what is calledlocked rotation with the earth (as are several other moons in our

solar system), and it is a long-term result of tidal forces between the

earth and moon. All the familiar seas and craters are on the moon’snear side; the far side was unknown until photographed by a Soviet

spacecraft in 1959.

Eclipses

In the first section of this book we saw how Starry Night wasable to simulate a solar eclipse. Both solar and lunar eclipses are

caused by the motion of the moon around the earth. When the moon

moves directly in front of the sun, there is a solar eclipse. When itpasses exactly opposite the sun in the sky, there is a lunar eclipse.

Eclipses are among the most awesome spectacles in nature,

and one of the first duties of astronomers thousands of years agowas to predict eclipses – or at least to be on hand to perform the

rituals that would cause the dreaded eclipse to hurry up and end!

Only since about 600 B.C. could astronomers predict eclipses withany accuracy. With Starry Night, you can see eclipses far into the

future and re-create eclipses that happened in the distant past. If

direction as the sun.The earth is never visible from the far side of the

moon – the side that permanently faces away from earth.Just as this side cannot be seen from earth, visitors to itwill never see the earth in their sky.


you select “Sky | Interesting Events”, you will see a list of all the

eclipses in the 20th and 21st centuries, and a description of whetherthe eclipse was solar or lunar, and whether it was total, annular or

partial. Choose “Best View” to view the eclipse from the prime

location or “Local View” to see what it looked like at that time foryour home location.

If you do search for eclipses, you will notice that they do not

happen randomly in time. The moon’s path is inclined 5° relative tothe sun’s path, and an eclipse can happen only when the sun is at or

near a node of the moon’s orbit – one of the two places where the

moon’s orbit crosses the ecliptic. When the sun passes a node, atleast one solar eclipse must happen, and there can be two. The sun

spends 38 days close enough to the node that the moon can pass in

front of it. If the moon eclipsesthe sun during the first 8 days of

that period, the eclipse will be

partial and the moon will returnand eclipse the sun again one

lunar month later, yielding two

partial eclipses. If the moon doesnot eclipse the sun until after the

8th day, there will be just one

solar eclipse, and it will be totalor annular, if seen from the best

viewing location.

Open the file “EclipseNodes” to see the geometry of the eclipsenodes. Two lines are marked. One is the path of the moon’s orbit;

the other is the ecliptic, the line which traces the sun’s apparent

path. The intersection point of these two lines is an eclipse node,and it is just to the bottom left of the sun. Step forward through time

in one day intervals and you will notice that the sun is moving closer

Eclipses can only occur whenEclipses can only occur whenEclipses can only occur whenEclipses can only occur whenEclipses can only occur whenthe sun is near the intersectionthe sun is near the intersectionthe sun is near the intersectionthe sun is near the intersectionthe sun is near the intersectionof the ecliptic and the moon’sof the ecliptic and the moon’sof the ecliptic and the moon’sof the ecliptic and the moon’sof the ecliptic and the moon’sorbit.orbit.orbit.orbit.orbit.

125The Moon

to the node. At the same time, the moon is rapidly moving aroundthe sky, but it is far enough away from the sun that it is off screen.

On August 8, the moon comes into view. At this time, the sun is

almost exactly at the eclipse node, so you can predict that there willbe a solar eclipse when the moon passes by the sun. Continue

stepping forward in time until August 11, when the moon and the

sun line up. As predicted, there is an eclipse. It is a total eclipse ifviewed from this location on Earth, which is Munich, Germany.

Thousands of years ago astronomers realized that a solar eclipse

could happen only during the 38-day period when the sun was nearone of the moon’s nodes, and this interval was called the eclipseseason. Before astronomers were capable of predicting eclipses

reliably, they issued warnings that an eclipse could happen duringthe eclipse season – just as today hurricane warnings are issued in

the Caribbean during the storm season. The predicted eclipse did

always occur, but it was often visible only from a distant part of theearth and they were ignorant of it (they counted it as a miss). Today

the only warnings astronomers issue regarding eclipses is to not

Eclipses in Ancient HistoryAncient eclipses are important to historians because

they allow us to date events with precision. The datesof many important events are poorly known, often onlyto the nearest decade, but the times of all solar andlunar eclipses can be calculated to the nearest minute.If an eclipse accompanied an historic event, the time ofthe event can be pinned down precisely. Eclipses providecrucial “anchor points” in history.


look at the sun without proper equipment.

As with the solar eclipse, both the sun and the moon must benear an eclipse node for a lunar eclipse to occur. During a solar

eclipse, the sun and the moon are at the same node, but during a

lunar eclipse, they are at opposite nodes, which are 180° apart inthe sky. Open the file “EclipseNodes2” to see this. You are looking

at the full moon, which is dark and in shadow because we are

witnessing a lunar eclipse. Double-click on the moon to open itsInfo Window and read its azimuth. Now scroll around in the sky to

find the sun. You will see that it is also at a node. If you open its

Info Window, you will see that its azimuth is about 180° removedfrom the moon’s. There must be at least one lunar eclipse each eclipse

season and it can be total, or there can be two – but then both are

partial.Eclipse season is the same for both solar and lunar eclipses.

After a solar eclipse, the moon must move about 180° in the sky for

a lunar eclipse to occur. The moon makes a complete 360° circle inabout four weeks, so it moves about 180° in two weeks. We would

therefore expect lunar and solar eclipses to be separated by about

two weeks and indeed, this is the case. Every eclipse season bringsat least one solar eclipse and at least one lunar eclipse.

If the moon crossed the sun’s path at the same point each year,

eclipses would happen on the same date each year, but it doesn’t, sothey don’t. The moon’s orbit regresses in a motion that is very similar

to the precession of the equinoxes (see chapter 3.3). The moon’s

two nodes precess westward along the ecliptic at the rate of 18.6°per year. The sun moves 18.6° along the ecliptic in 18.6 days and

arrives at the node earlier the next year, causing “eclipse season” to

move backward through the calendar. The eclipse of December 14,2001 is followed by an eclipse on December 4, 2002.

127The Moon

The Last Total Solar EclipseThe earth has enjoyed total solar eclipses since the

moon was formed, but our distant ancestors will notsee any. Because of tidal friction with the earth, themoon is receding from us at the stately rate of 4centimeters (1.5 inches) per year – about the same ratethat the continents drift or that your fingernails grow.The moon’s apparent diameter is shrinking as its distanceincreases. In about the year 600,000,000 AD the moonwill totally eclipse the sun for the last time; after thistime the moon will be so distant that its disk will nolonger be large enough to completely cover the sun,and people will see only annular and partial eclipses.

Viewing a Lunar Eclipse

A lunar eclipse is a direct (although less visually spectacular)counterpart of a solar eclipse. The sun is eclipsed when the moon

moves in front of it and we find ourselves in the moon’s shadow.

The moon is eclipsed when it moves into the earth’s shadow. Thishappens only at full moon – when the moon is directly opposite the

sun.

The earth’s shadow – like all shadows – has a central umbraand an outer penumbra. The umbra is the place where, if you stand

there, the sun is totally blocked and the eclipse is total. The penumbra

is the place where the eclipse is partial. A lunar eclipse has stagesthat resemble a solar eclipse, and tables of eclipse circumstances

will list the times of the several “contacts” (when the moon begins

to enter the penumbra, is fully within the penumbra, enters the umbra,


is fully within the umbra, mid-eclipse, begins to leave the umbra,

begins to leave the penumbra, and is finally out of the penumbra).Starry Night will show these several stages graphically and you can

determine the timings from the date/time display. Open the file

“LunarTotal” and step through time to view a total lunar eclipse.Note the faint penumbra and the darker umbral shadow which the

moon passes through.

During a shallow penumbral eclipse, when the moon passesthrough only the outer edge of the earth’s shadow, the moon may

not darken noticeably. In a deep penumbral eclipse the darkening is

barely noticeable even to a careful observer. In an umbral eclipse,one portion of the moon grows dark and a casual observer would

notice that something is amiss. Only during a total eclipse does the

moon darken substantially and take on a reddish or orange color.This coloring comes from light refracted around the edge of the

earth towards the moon; it comes from all the earth’s sunrises and

sunsets. If the earth’s atmosphere is especially opaque, as happensfollowing a major volcanic eruption, the eclipse can be so dark that

the moon disappears, but this is rare. Generally the moon takes on a

deep coppery color, dims as the stars come out, and looks very pretty.To superstitious people in former times, the red color of the moon

made it an unpleasant and fearsome omen.

The Greek’s Proof of a Round EarthThe ancient Greeks knew that the earth is a sphere

by observing lunar eclipses. They saw that the edge ofthe earth’s shadow on the moon always has a circularshape, and they knew that the only object that onlycasts a circular shadow is a sphere. They did not, however,know the size of the earth or moon or the distance tothe moon.

129The Moon

The earth casts a much larger shadow on the moon than the

moon does on the earth, because of the earth’s larger size. Thismeans that many more people will see a total lunar eclipse than a

total solar eclipse for three reasons:

1) total lunar eclipses are slightly more common than totalsolar eclipses (there are about 3 total lunar eclipses for every 2

total solar eclipses) because the earth, moon, and sun do not

have to be exactly in a straight line for a total lunar eclipse tooccur.

2) most total lunar eclipses are total as seen from almost

anywhere on earth where the moon is above the horizon at thetime of the eclipse, while total solar eclipses are only total as

seen from a narrow path.

3) total lunar eclipses generally last much longer than totalsolar eclipses.

Unlike a solar eclipse, you need take no special precautions to

observe a lunar eclipse. The moon is a dark rock sitting in space,and when eclipsed it just grows darker still. Use binoculars or a

telescope to bring out subtle coloring.

Occultations

Another interesting event to observe with binoculars or a tele-scope is the occultation of a planet or bright star by the moon. As

the moon circles the earth, it occasionally passes in front of a more

distant object and “eclipses” it. If the object eclipsed has a muchsmaller apparent size than the moon (a planet or star, for example,

but not the sun), then the preferred astronomical term is “occulta-

tion,” from the Latin root “occult,” which has nothing to do withthe supernatural but simply means “to conceal or hide.”


A star, whose apparent diameter is miniscule, blinks out

instantaneously when the moon’s edge moves in front of it, and it isstartling how abruptly a star disappears from sight. Only stars within

5° of the ecliptic can be covered by the moon, so we can only see

star occultations for a handful of bright stars (Spica and Regulusare the most prominent).

A planet takes several seconds to disappear as the moon moves

in front of it. Up to an hour and a half later, the star or planet reappearsfrom behind the other side of the moon. Depending on the moon’s

phase, the leading edge will be light while the trailing is dark (full

moon to new), or vice versa (new moon to full). All planetseventually cross the ecliptic, so any planet can be involved in a

planetary occultation.

Both types of occultations are shown in the files “Occultation1”and “Occultation2”; Open these and step forward through time

slowly to watch Saturn and the star Spica respectively disappear

and then reappear. The more readily visible occultations are listedin monthly sky calendars such as the one appearing in Sky &

Telescope magazine, and at the International Occultation Timing

Association’s web page (see www.livesky.com for the link to thissite).

4.2

PLANETS, ASTEROIDS &

COMETS

Planets

Chapter 1.3 of this book showed us a few fundamental aspects

of the planets in our solar system: the earth is a planet like the othersin motion about the sun; all of the planets (with Pluto the major

exception) revolve around the sun in the same plane; and the planets

move in the same direction but at different speeds. Humans haverequired a significant fraction of their collective history to arrive at

these ideas. It was especially difficult to discard the idea of the

stationary earth.In the exercise for chapter 1.3, we viewed the motion of the

planets with the sun at the centre, which we now know is the correct

view. But how would the planetary motions look if the Earth was


really stationary? To find out, click on the “Go” menu and select

“Earthcentric”. Now the motions are not so simple! The sun makesa regular loop around the earth, but the other planets have more

complicated motions. It is easy to see why it was thought from the

time of the ancient Greeks until about 1600 AD that the planetsmove on epicycles – circles on circles – as they orbit the earth.

As seen from Earth, each planet repeats a regular pattern in the

sky. This cycle can be thought of as beginning when the earth, sunand the planet are all in a straight line, with the planet on the opposite

side of the sun from the Earth (this alignment is called superiorconjunction), and ends the next time they come together in the same

Kepler’s LawsPrior to the work of Johannes Kepler (1571 - 1630),

no one understood how the planets orbit the sun or therelationships between their orbits. Kepler had the goodfortune to inherit the exceptionally precise planetaryobservations of his teacher, Tycho Brahe, and theyallowed him to devise his three “laws of planetarymotion.” They summarize how the solar system is puttogether and how it operates. In simple terms, the lawsstate:

1. Planetary orbits are ellipses with the sun atone focus.

2. Planets orbit fastest when closest to the sun.3. The length of a planetary year is related to its

distance from the sun by a simple formula.Before Kepler, planet motions were a mystery; after

Kepler, the planets’ positions could be predictedaccurately far into the future.

133Planets, Asteroids & Comets

alignment. The length of this cycle depends not on the period of the

planet’s orbit, but on the relative motions of both the earth and theplanet about the sun. Mars actually takes the longest to complete its

cycle, about 26 months from superior conjunction to superior

conjunction. During this cycle, a planet’s phase and brightness willboth vary, as will the best time to observe the planet. But how much

these quantities vary depends on the particular planet. So let’s take

a look at the cycles of several different planets now.

The Inferior Planets

We’ll begin with the two inferior planets, to use a term that

sounds disparaging but that is only a holdover from an era when all

that distinguished one planet from another was its orbit. The inferiorplanets are Mercury and Venus, and they stay in the vicinity of the

sun as seen from the earth. They can appear to the east (or left) of

Orbit of Mercury or Venus

Earth's orbit

SunEarthInferiorConjunction

SuperiorConjunction

Greatest easternelongation

Greatest western elongation

ImportantImportantImportantImportantImportantpoints alongpoints alongpoints alongpoints alongpoints alongthe orbits ofthe orbits ofthe orbits ofthe orbits ofthe orbits ofMercury andMercury andMercury andMercury andMercury and

Venus, theVenus, theVenus, theVenus, theVenus, theinferior planets.inferior planets.inferior planets.inferior planets.inferior planets.


A planetary transit ofA planetary transit ofA planetary transit ofA planetary transit ofA planetary transit ofVenusVenusVenusVenusVenus

Planetary TransitsWhen Venus or Mercury is at inferior conjunction, it

may make a transittransittransittransittransit in front of the sun. Planetary transitsare actually a type of partial solar eclipse! Mercury andVenus are too small to eclipse the sun when they movein front of it, and we see them cross the sun’s surface asslowly moving black dots. Mercury appears the size of asmall sunspot and Venus the size of a large sunspot, sotransits cannot be observed without properly filteredtelescopes capable of fairly high magnification.

Just as solar eclipseshappen only during “eclipseseason” (see chapter 4.1)when the moon’s orbit is soaligned that the moon canpass in front of the sun, sotoo planetary transitshappen only when theplanet’s orbit is aligned so that the planet can “eclipse”the sun as seen from earth. For this reason, transits arerare, and they occur in sets separated by wide intervalsof time. Transits of Mercury occur alternately in May orNovember in 2003, 2006, 2016, 2019, 2032, 2039, etc.They last up to 7-1/2 hours. The only two transits ofVenus in the 21st century are in June 2004 and June2012. If you are lucky enough to watch a transit, you willsee the planet slowly creep across the sun from east towest, passing any sunspots that are present as it goes.Open the file “VenusTransit” and step through time towatch the transit of Venus in 2004. Change the locationto see if it is visible from your home town.


the sun, in which case they are visible in the evening sky; to the

west (or right) of the sun, in which case they are visible in themorning sky; behind the sun, in which case they cannot be seen

(superior conjunction); or at inferior conjunction when they lie

between the earth and sun (which no other planets can be), and inwhich case they also cannot be seen. The ease with which these

planets can be viewed is in direct correspondence with their angular

separation from the sun. The farther a planet appears from the sun,the longer it is above the horizon during the night. Venus gets far

enough from the sun to be conspicuous much of the year, but

Mercury never travels far from the sun, so it usually can’t be seenat all.

Mercury never strays more than 28° from the sun. As seen

from the earth’s equator it never sets more than two hours after thesun or rises more than two hours before it. This number is smaller

the farther you are from the equator, making Mercury even harder

to see from mid-latitudes. The bottom line: Mercury is seen onlynear the horizon during twilight. It is best to look for it with binocu-

lars and it is most easily found when the thin crescent moon is near

it to guide the way.Open the file “Mercury” to follow Mercury through several

orbital cycles, beginning with the planet at superior conjunction

(on the far side of the sun) on May 18, 2006. The orbital path whichMercury makes around the sun is shown. Mercury is so nearly in

line with the sun that you will have to zoom in to see it. In reality, it

rarely is exactly behind the sun (just as it rarely passes exactly infront of the sun) because its orbit is tilted 7° to the ecliptic. This is

more than any planet but Pluto. Mercury is almost invariably north

or south of the sun when in conjunction with the sun. Center andzoom in on Mercury to confirm that it is a tiny but full disk (you

will have to zoom in very close, to less than 1'), and then zoom

back out to the standard 100° field of view.


Step through time in one-day intervals and watch Mercury

swing around to the left (or east) of the sun. Its angular separationfrom the sun increases daily until it reaches its maximum angular

separation from the sun on June 23. This is called its greatest east-ern elongation, which can be as little as 18° or as much as 27°.Zoom in on Mercury to verify that it is at third-quarter phase. The

planet is so tiny – typically 5 arcseconds in diameter – and so near

the horizon that its phase is hard to see even with a good telescope.Restore the field of view to 100° and change the time to 8 p.m. to

see that it shines near the western horizon. Change the time step to

3 minutes and step forward in time to watch it set at 9:43 pm.Mercury is visible only during a narrow window of time, after the

sky becomes dark enough to see it, but before it sets.

Change the time back to 12:00 p.m. and the time step to 1 day.Start time running forward again. Mercury remains near its greatest

eastern elongation for a few days, and its position in the sky changes

little because it is moving nearly directly towards the earth then,although it often moves a short distance north or south (in this

instance) in a shallow arc. Then Mercury begins to move westward

toward the sun again. When it passes the sun, it is on the near sideof the sun and much closer to the earth than when on the sun’s far

side, and Mercury’s apparent speed across the sky is greater than

when it was on the far side of the sun. It is at inferior conjunction,when it is most nearly between the earth and sun, on July 18, and

this is the date when it officially passes from the evening to the

morning sky. It is now at its new phase (zoom in to verify, and thenzoom out again), but you won’t see it in the sky because it rises and

sets with the sun. A week later it reappears as a “morning star” near

the eastern horizon during dawn. Telescopically it is a large (byMercury standards) thin crescent. It climbs higher each day until it

reaches its greatest western elongation on August 7. This coin-


cides closely with its earliest rising time and its best pre-dawn

visibility. At its greatest elongation it is at first-quarter phase.Mercury then moves more slowly back towards the sun and

disappears before it is in line with the sun at its superior conjunction

on September 1, when it is full again.Mercury reveals little through a telescope. Ironically, it is one

of the three planets whose surface is “visible” (Mars and Pluto are

the others; the rest of the planets are shrouded by clouds), but it istoo small and too distant for any surface features to be seen. Only

its phase might be seen, and although it changes phase rapidly, the

blurring effect of turbulence in our atmosphere (a problem with allobjects observed at low altitude) makes it hard to tell what phase it

is even through a good telescope.

Venus goes through a cycle similar to Mercury’s. Open the file“Venus” and step through time in 3-day steps to follow a typical

cycle of Venus, beginning at superior conjunction on October 27,

2006. Note how the orbital path of Venus takes it much farther fromthe sun than Mercury’s path does. Zoom in at any time to see the

phase of Venus. When it is on the far side of the sun it is small

(about 10 arcseconds in diameter) and full. The earth and Venus aremoving around the sun at similar speeds (30 vs. 35 km per second,

or 18 vs. 22 miles per second respectively), so Venus increases its

angular separation with the sun only slowly. When it is 10° or morefrom the sun, it can first be seen as the “evening star” low in the

west after sunset. Its great brilliance makes it easy to spot despite

its low height. It is now waxing gibbous. Months later it reaches itsgreatest eastern elongation (June 9, 2007), which can be as great as

47° from the sun. At this time in its cycle, Venus sets after 11 p.m.

at mid-northern latitudes and remains beautiful in the west longafter twilight ends. It remains near its eastern elongation for weeks

while it shines brightly late into the evening; it is then heading


towards the earth and moving very slowly against the stars. During

the next several weeks it catches and then passes the earth on aninside orbit, quickly moving toward inferior conjunction with the

sun (August 18, 2007). During these final weeks of its evening

appearance, it becomes an increasingly thin and increasingly largecrescent up to an arcminute in diameter – so large that its phase can

be seen in very good tripod-mounted binoculars. It then seems to

drop out of the evening sky, and in one month its visibility dropsfrom very conspicuous to hard to see.

After passing inferior conjunction, Venus quickly reappears in

the morning sky. It gains altitude rapidly day by day until it reachesits greatest western elongation (October 28, 2007), when it is half-

full. Its disappearance from the morning sky is as slow as its

appearance in the evening sky because it is on the far side of the sunand its motion relative to the sun is slow.

Venus is by far the brightest planet. Its brilliance comes from

Galileo and the Phases of VenusGalileo discovered the phases of Venus with one of

his first telescopes, demonstrating that Venus orbits thesun, rather than the earth. If Venus orbits the sun, socould the earth, and this was important proof of theCopernican theory of a sun-centered solar system, whichuntil then had been a mathematical curiosity. Galileopublished his observations in code to establish thepriority of his discovery until he could confirm it. Thefile “VenusGalileo” shows Venus as Galileo saw it fromFlorence, Italy, in October 1610 through his telescope. Itwas near full and quite small, and it was not until monthslater that Galileo could easily discern its non-round shape.


thick and highly reflective clouds that permanently shroud the planet.

The surface features of Venus cannot be seen through any telescope.In fact, the clouds themselves are featureless in wavelengths visible

to the eye. The planet is so brilliant it can be seen during the daytime

if you know exactly where to look.

The Superior Planets

The other planets, Mars through Pluto, remain outside the earth’s

orbit and are called superior planets. They are only superior in an

orbital sense. Unlike the two inferior planets, the superior planetsnever come between the earth and sun. Also unlike the inferior

planets, whose visibility is limited to the hours just before sunrise

or just after sunset, the superior planets can lie opposite the sun andcan be visible at any time of the night.

Earth's orbit

Orbit of Mars, Jupiter, Saturn, etc.

SunEarth

ConjunctionOpposition

Important pointsImportant pointsImportant pointsImportant pointsImportant pointsalong the orbits ofalong the orbits ofalong the orbits ofalong the orbits ofalong the orbits of

the superior planets –the superior planets –the superior planets –the superior planets –the superior planets –Mars through Pluto.Mars through Pluto.Mars through Pluto.Mars through Pluto.Mars through Pluto.


Open the file “Mars” to follow one Martian cycle. We begin

with the planet at superior conjunction on the far side of the sun onJuly 1, 2000. Mars is almost 1° north of the sun. Zoom in to see that

it is at full phase – but the superior planets are always near full and

can never be seen as crescent. Zoom back out and step forwardthrough time in 1-day intervals. Mars is moving very slowly east-

ward against the distant stars, but the sun is moving faster and leaves

Mars behind. Unlike the inferior planets, which move faster thanthe sun and reappear east of the sun in the evening sky after their

superior conjunction, Mars loses ground and reappears west of the

sun in the morning sky. The relative speed of the sun and Mars isvery low, and it takes at least a month for Mars to become far enough

west of the sun to be visible above the horizon in morning twilight.

Because it is on the far side of the sun, it is tiny as seen through atelescope and not especially bright at 2nd magnitude, so its cycle

does not begin dramatically.

On January 1, 2001, change the time to 4 a.m. Resume runningforward through time and notice that each morning Mars is higher

at 4 a.m. than the morning before. This is because it rises a few

minutes earlier each day. Notice that Mars is a few degrees north ofthe red star Antares, the “rival of Mars,” early in March.

Around March 15, change the time to 2 a.m. and resume

stepping forward through time to notice an interesting change inthe motion of Mars. In April, 2001, Mars begins to slow noticeably

in its eastward motion against the stars, and on May 15 it stops, is

briefly stationary, then reverses its course and begins headingwestward in retrograde motion. The planet does not actually move

backward, of course; it is an optical illusion, but an effective one.

During the several months that the earth, which is traveling fasteron an inside path, takes to pass Mars, Mars appears to move

backwards against the stars. It is the same effect you get when you

pass a slower moving car on the highway – in the time it takes you


to pass it, the other car seems to move backwards in relation to the

distant trees and hills, although both of you are moving forward atdifferent rates.

Mars moves westward in retrograde motion at an accelerating

rate through the summer. The earth passes Mars on June 13, 2001,and then the red planet is at opposition; it lies opposite the sun.

Mars rises at sunset and is visible all night long. It is at its closest,

67.3 million kilometers (41.8 million miles) and at its brightest aweek later. (The date of opposition and closest approach would

coincide if the two orbits were perfectly round, but their non-

circularity causes these dates to differ by a few days.). At this time,Mars is about four magnitudes brighter than it was at superior

conjunction, or 40 times brighter! Mars and Venus vary in brightness

much more than any other planets. You should be able to figure outwhy this is so.

Following opposition, Mars begins to fade as the earth leaves

it behind. It sets earlier each day. It continues its westward, retrogrademotion until July 19, 2001, and then it seems to stop against the

stars again and resume its normal eastward motion. Mars retrogrades

toward Antares during early July, but reverses itself before reach-ing it. On July 1, 2001, change the time to 10 p.m., and resume

running forward through time.

Zoom in during the late fall of 2001 and notice that Mars isnearly full, but not 100% so. This gibbous shape is as far from being

full as an outer planet can be.

Mars moves eastward at an accelerating rate, crosses the eclipticon February 10, 2002, and moves into superior conjunction with

the sun again on August 10, 2002. If you want to follow the path of

Mars until it is again at superior conjunction, you will have to setthe time back by a few hours in March or April 2002.

Each time the earth passes Mars, Mars is not equally close

because the two planets’ orbits are not circular. The closest oppo-


sitions in the first third of the century come in August, 2003; July,

2018, and September 2035.Telescopically, Mars is a challenging object. Even when closest

to earth it is surprisingly – and disappointingly – small, and high

magnification on a good telescope with a steady atmosphere arerequired to see much. Practice is essential too – you will not see

much at first glance, even on an ideal night. Allow time for your

eye to pick out subtle details. The polar caps are bright and theirwhiteness contrasts well with the orange deserts that make up most

of the planet’s surface. The giant Hellas basin, when filled with

white clouds, can rival the polar caps in brilliance. Indistinct darkmarkings are visible under ideal conditions and make the Mars afici-

onado wish for a larger telescope so as to see more.

Mars rotates in 24 earth hours and 37.5 earth minutes, so youcan watch it rotate significantly during the course of an evening.

Each night we see it turned slightly from its orientation at the same

time on the previous evening. Open the file “MarsZoomed” andstep forward at one day intervals to watch new features rotate into

view while others rotate out. Each night surface features are offset

by 37 minutes from the previous night, and during the course of amonth you can see the entire surface. You will also notice Mars’

two moons (Phobos and Deimos) come in and out of view. These

moons revolve very rapidly - Phobos takes seven hours to completean orbit of Mars, while Deimos takes 30 hours. These moons are

visible only in very large telescopes.

Jupiter’s path resembles Mars’ path (and all the other superiorplanets), although its cycle is shorter. Open the file “Jupiter”. We

begin with it on the far side of the sun on July 19, 2002. Step forward

through time at one-day intervals, and change the time of day whennecessary in order to keep Jupiter above the horizon. The sun seems

to race away, leaving it behind, and Jupiter first appears in the

morning sky less than a month later, where its great brightness


(second only to Venus) makes it relatively easy to see. It rises four

minutes earlier each day (the same as the stars, since Jupiter’s motionagainst the stars is so very slow) and eventually it rises before

midnight and moves into the evening sky (October 26, 2002). On

December 5, 2002, the earth begins to catch Jupiter, and Jupiterbegins its westward retrograde motion. The earth and Jupiter are

closest on February 2, 2003, which is also when it is at opposition

(to within a few days). Jupiter ends its retrograde motion on April5, 2003, resumes moving eastward, and is still moving eastward

when it disappears into the glare of the sun in August. It is at superior

conjunction on August 21, and the 13-month cycle is completed.Jupiter’s great distance means that its size and brightness change

little during its orbit, and it remains impressively large compared to

the other planets. Even a small telescope will show a few cloudbands aligned parallel to its equator, and perhaps the Great Red

Spot, which is a storm in Jupiter’s atmosphere. A good telescope

will show more features than a person can sketch. The planet’s rapid10-hour rotation allows you to see the entire surface over the course

of one long night. The file “JupiterZoomed” shows the planet’s

rotation at 5-minute intervals over a long winter night.Visible in this file are Jupiter’s four major moons. These moons

are a constant delight to

amateur astronomers, andStarry Night will show the

four major ones, which were

discovered by Galileo. Theycan be seen in any telescope

and even in binoculars. They

orbit the planet so quickly(1.68 days for Io to 16.7 days

for Callisto) that they change

appearance night by night. The gas giant Jupiter. The Great RedThe gas giant Jupiter. The Great RedThe gas giant Jupiter. The Great RedThe gas giant Jupiter. The Great RedThe gas giant Jupiter. The Great RedSpot is visible on the lower right.Spot is visible on the lower right.Spot is visible on the lower right.Spot is visible on the lower right.Spot is visible on the lower right.


Their motion can be seen almost minute by minute if they are next

to the planet or to each other. Amateurs with decent telescopes enjoywatching the moons pass in front of Jupiter, cast their shadows on

Jupiter (the shadows are dark and contrast well against the bright

white clouds), disappear in eclipse in the planet’s shadow, becomeocculted by the edge of the planet, or even (very rarely) eclipse

each other. Jupiter’s moons eclipse and occult each other when the

plane of their orbit is seen edge-on to our line of sight, and thishappens in June 2003, April 2009, Nov. 2014, March 2021, and

Oct. 2026. Change the date in the “JupiterZoomed” file to the current

date, if Jupiter is visible tonight, to see the changing configurationof its moons.

The motions of Saturn and the other outer planets are similar

to Jupiter’s and will not be treated here. You can use Starry Night tofollow their motions many centuries into the future. Saturn, how-

ever, has rings and moons that set it apart.

Saturn’s rings are one of the spectacles of the night sky whenseen through a decent telescope. Several rings orbit the planet,

nowhere touching it, and they have different brightnesses and widths.

The rings are composed of grains of ice and rock. The planet itselfis quite bland, but the rings make up for the lack of clouds.

Saturn’s rings wrap around the planet’s equator. Saturn’s pole

is tilted relative to its orbital plane, and as the planet orbits the sunwe see its rings at a

c o n s t a n t l y - c h a n g i n g

orientation that followsSaturn’s 30-year orbital

period. When Saturn’s pole is

tilted the greatest amounttowards or away from the

earth, as it is in 2002 and

2017, we see the rings “open”to our view with a tilt of 27 The rings of SaturnThe rings of SaturnThe rings of SaturnThe rings of SaturnThe rings of Saturn


degrees; they are wide and bright. Half-way between these dates,

the rings are briefly edge-on to our view, and then they almostdisappear. They are edge-on in September 2009 and March 2025,

but the planet is behind the sun on both these occasions and we will

miss the novelty of seeing Saturn without its rings. We will notactually see a ringless Saturn until 2038. Open the file

“SaturnRingTilt” and run forward through time to see how the

appearance of Saturn’s rings will change over the next few decades.Saturn’s moons are more challenging than Jupiter’s major four,

but huge Titan can be seen in any telescope and even a modest

telescope will show one or two others. Ninth-magnitude Titan orbitsSaturn in 16 days, and it lies five ring-diameters from Saturn when

east or west of the planet. Rhea, Dione, and Tethys are between 10th

and 11th magnitude and orbit even more quickly, closer to the planet.Uranus and Neptune are bright enough to see in binoculars or

small telescopes, and Uranus has been spotted with the naked eye,

but the trick is to know where to look. Their tiny greenish disksdistinguish them from stars under high magnification (which is how

William Herschel discovered Uranus), but they show no surface

features and their moons are visible only with large telescopes. Theymove slowly from constellation to constellation and hold little

interest for amateur astronomers. Use Starry Night to see if they are

visible tonight.Pluto looks like a 14th magnitude faint star, and it can be spotted

only in a very large amateur telescope. Pluto is much fainter than

the faintest stars Starry Night Backyard can show, so if you wish tofind Pluto with your giant telescope, use Starry Night Backyard to

find Pluto's coordinates and then mark its position on a detailed star

chart. Starry Night Pro owners can just make their star charts withPro, as it does include enough stars fainter than Pluto to make

detailed charts. Confirm that you have found it when you record its

slow movement against the stars from night to night.


Conjunctions

As the planets orbit the sun and move across the sky, one

occasionally passes another, and when this happens it is called aconjunction. Conjunctions happen almost every month and are not

rare, but some are more interesting than others. Some are spectacular.

A conjunction is defined as the moment when two planets havethe same ecliptic longitude (see the section on “Co-ordinate

Systems” in chapter 2.2 for a definition of ecliptic longitude).

Alternately, it is the moment when two objects have the same rightascension, which is slightly different. Neither is necessarily the exact

moment when they are at their closest. A conjunction looks different

from different locations on earth because of parallax. Theconjunctions (superior and inferior) which we looked at in the above

section all involved the sun and another planet, but the most

interesting conjunctions do not involve the sun, but instead two ormore planets, or one or more planets and the moon.

Mercury and Venus move so quickly that they are involved in

most conjunctions that occur. Slow-moving Saturn, in contrast,seldom passes another planet, but is itself often passed. The moon

is in conjunction with each planet once a month, and if it comes

near the bright planet Venus, Mars, or Jupiter, it can be a spectacularnaked-eye sight. You can discover conjunctions to your heart’s

content by running Starry Night through time.

A conjunction is interesting to amateur astronomers when thetwo objects come close enough to each other to be visible at the

same time in a pair of binoculars or, exceptionally rarely, through a

telescope. Even rarer is an occultation of one planet by another.None will occur within the lifetime of most people now alive. You

will find a sampling of mutual planetary occultations in the files for

this chapter; those included took place in 1590, 1771, and 1808. At12:45 on November 22, 2065 the centers of Venus and Jupiter will


be 16 arcseconds apart and the northern edge of Venus will pass in

front of Jupiter! Mark it on your calendar or preview it by openingthe file “Conjunction 2065”.

Three or more planets cannot be in conjunction simultaneously,

although the word is sometimes misapplied when several are closeto each other. Such a grouping of planets is called a massing, and

you can discover massings with Starry Night. The best in history

was in early 1953 BC, and it was profoundly important to the ancientChinese, who recorded and remembered it. Open the file “Massing

1953 BC” and step

forward at 1-day inter-vals through the

month of February to

see a truly amazingsight develop in the

predawn sky! There

are files with thischapter for planetary

massings on

September 29, 2004;December 10, 2006; and May 11, 2011 (“Massing 2004”, “Massing

2006”, and “Massing 2011” respectively).

A popular misconception is that all the planets sometimes lineup like billiard balls in a row, and dramatic images of such an

alignment are even seen on book covers. The reality is different.

Jean Meeus, the foremost thinker about planet alignments, hascalculated how close the planets come to being in a line. He finds

that, between the years 3100 BC and 2735 AD – a time span which

includes all of recorded history – the minimum separation of thefive naked-eye planets was 4.3° on February 27, 1953 BC (the

massing we just looked at in the file “Massing 1953 BC”). The best

groupings closest to the present are: April 30, 1821 (19.7°); Febru-

The massing of all five naked eye planetsThe massing of all five naked eye planetsThe massing of all five naked eye planetsThe massing of all five naked eye planetsThe massing of all five naked eye planetsin 1953 BC.in 1953 BC.in 1953 BC.in 1953 BC.in 1953 BC.


ary 5, 1962 (15.8°); May 17, 2000 (19.5°); and September 8, 2040

(9.3°). The 2040 grouping, which includes the crescent moon at noextra charge, will be spectacular (examine it in advance by opening

the file “Massing 2040”).

If you consider the three outermost planets too, all the planetsnever line up.

A triple conjunction happens when a planet makes its retrograde

loop near a star; the planet passes that star once in its normal forwardmotion, a second time when in retrograde motion, and a third time

when it resumes its forward motion. Each outer planet is in triple

conjunction with all the stars within the bounds its retrograde loop,but occasionally a bright star (or even another planet!) is within

that loop. (The object does not have to be within the loop itself, but

within the limits of the loop in Right Ascension or ecliptic longitude.)One famous triple conjunction is the triple conjunction of Jupiter

Star of BethlehemThe famous Star of Bethlehem, seen so often on

Christmas cards and topping off Christmas trees, mayhave been a conjunction of planets. Astrologers of thetime looked to the sky for omens, and one of the bestwas a conjunction of bright planets. In the summers of3 and 2 BC – the years when Jesus is most likely to havebeen born – there were two very close conjunctions ofJupiter and Venus (Venus actually occulted Jupiter asseen from South America in the second conjunction)and three conjunctions of Jupiter and the star Regulus.It is plausible that the wise men, who were astrologers,would have interpreted so interesting a series of con-junctions as the fulfillment of ancient prophecies.


and Regulus that is associated with the star of Bethlehem. Open the

file “ConjunctionChristmas”. You begin on August 1, 3 BC. FollowJupiter and Regulus during the following ten months, adjusting the

time of day as necessary to keep Regulus above the horizon.

Some people whose knowledge of astronomy is limited areconcerned that when planets align, their gravity is somehow

magnified and they exert a major pull on the earth. The gravitational

or tidal force of a planet is completely insignificant, and even whenseveral are massed together it has no effect on our planet or on

ourselves. Planetary alignments do not cause cosmic disasters.

Minor Bodies: Asteroids and Comets

On New Year’s Day, 1801 – the first day of the 19th century –the Italian astronomer Giuseppe Piazzi discovered a new “planet”

between the orbits of Mars and Jupiter. It was much smaller and

fainter than the other known planets, so it was called a minor planetor asteroid (from its starlike appearance). Since then, over 10,000

asteroids have been discovered and they are a fascinating group of

objects.Asteroids are fragments of small planets that were shattered in

collisions with each other early in the history of the solar system.

Jupiter’s gravity prevented the formation of one big planet insideits orbit, and the many asteroids are the result. Most asteroids orbit

within the asteroid belt – a wide zone between the orbits of Mars

and Jupiter – but many others have been knocked out of the asteroidbelt during subsequent collisions and wander throughout the solar

system.

The largest asteroid is Ceres, which has a diameter of 930 kilo-meters (580 miles). It and all the other large asteroids remain safely

within the asteroid belt, but small ones, which can drift throughout

the solar system, can strike the earth. When one does, it falls through


the atmosphere as a meteor, and if it survives to reach the ground it

is called a meteorite. Meteorites are fragments of shattered planets.The impact of a giant asteroid fragment (or comet fragment) is

widely credited for the extinction of the dinosaurs 65 million years

ago. Most meteorites are stone, but a few are made of iron.The largest asteroids can be seen with binoculars or a small

telescope if you know where to look. Starry Night will show asteroids

in the Planet List, and you can centre and lock on them or turn ontheir orbits like any other object. Open the file “Asteroids” and start

time running forward to see the orbits of 5 major asteroids, all of

which are between Mars and Jupiter in the asteroid belt. Unlike theorbits of the planets, the orbits of these asteroids (particularly Pallas)

are tilted at large angles to the plane of the ecliptic. The thrill in

observing asteroids is simply to find and track them. Dozens arewithin the range of amateur equipment.

The planets and minor planets circle the sun with great regularity

and we can know exactly where they will be thousands of years inthe past or future. This is not true for comets. Most of them have

not yet been discovered – and won’t be for thousands of years!

Comets were born in the outer fringes of the solar system, andthat is where nearly all of them still reside. There is a huge reservoir

of billions of giant ice balls beyond the orbit of Neptune, of which

Pluto is the largest member. If these ice balls stay there, we will notknow of them (except by diligent searches conducted with large

telescopes) and they will not bother us. However, occasionally one

is deflected out of its distant but stable orbit into a new orbit thatbrings it into the inner part of the solar system, and then we see it as

a comet. Comet Hale-Bopp, for example, was first sighted near the

orbit of Jupiter in 1995, about two years before it was at its brightestand closest to the sun.

A comet is a chunk of ice and dust only a few kilometers across.

The ice is mostly frozen water and carbon dioxide and the dust is


simple silicates. When far from the sun, the ice is frozen and the

comet is too small and too faint to be seen. But as the cometapproaches the sun, it warms. Sunlight thaws the ice, which

evaporates (it does not melt – there are no puddles in space!),

carrying with it the dust that was embedded in it. The gas and dustform tails that can be long and beautiful. Sunlight causes the gas to

fluoresce while the dust merely reflects sunlight, and the gas tail

often has a different shape and colour than the dust tail. A comet isbrightest when it is closest to the sun, which is when it is warmest,

and that is when it is traveling the fastest. If it is near the earth at the

same time, it can zip across our sky at the rate of several degrees aday. As it leaves the sun’s vicinity, it cools, becomes fainter, and

slows down. It may not return for hundreds or many thousands of

years. Some comets have been captured into short orbits and appearevery few years, but these comets have had their ice evaporated

after spending so many years near the sun and they are exhausted,

and all are faint, with the exception of one – Halley’s. Halley’s Cometis the only bright comet that returns often enough for each person

to have a chance to see it, and many

people have. It returns approximatelyevery 76 years, most recently in 1986.

Like any other comet, Halley’s loses ice

through evaporation every time itapproaches the sun, and it is not as bright

now as it was the first time it passed

through the inner solar system.Several prominent comets are shown

in the Planets List in Starry Night. Open

the file “Hale-Bopp” and step throughtime to see the passage of the famous

comet near its peak brightness back in

1997. Now open “Hale-Bopp 2”. Step

Starry Night’sStarry Night’sStarry Night’sStarry Night’sStarry Night’srepresentation of Cometrepresentation of Cometrepresentation of Cometrepresentation of Cometrepresentation of CometHale-Bopp. The gas tail,Hale-Bopp. The gas tail,Hale-Bopp. The gas tail,Hale-Bopp. The gas tail,Hale-Bopp. The gas tail,is quite narrow, whileis quite narrow, whileis quite narrow, whileis quite narrow, whileis quite narrow, whilethe dust tail is morethe dust tail is morethe dust tail is morethe dust tail is morethe dust tail is morewidely scattered.widely scattered.widely scattered.widely scattered.widely scattered.


Meteors and Meteor ShowersMost meteors come from comets. Comets shed dust astheir ices evaporate, and this dust never returns to thecomet. The dust drifts around the sun, following theorbit of the comet. If a dust particle is swept up by theearth, it falls through the atmosphere towards the ground.Friction with air molecules heats it to incandescence andit bursts into flames; we see it as a meteor. Alternatepopular terms are “falling star” or “shooting star.”If the earth passes through or near the orbit of a comet,we pass through a region filled with dust, and meteorsfall by the hundreds. A shower happens at the sametime each year, when the earth returns to that part of itsorbit. The shower can last for a few hours to a fewweeks. Listed below are the best meteor showers, theirpeak dates, and the number of meteors per hour anobserver in a dark location might optimistically see:

Shower Date Hourly RateQuadrantid January 3 85Eta Aquarid May 5 30Delta Aquarid July 29 20Perseid August 12 100Orionid October 22 20Geminid December 14 100Ursid December 22 45

The Eta Aquarid and Orionid showers come from Halley’sComet on the inbound and outbound legs of its orbitrespectively. The Geminids come from the asteroidPhaethon.


through time to watch the comet approach the sun and then fly away

again. Note how its orbit is very different from the near-circularorbits of the planets.

New bright comets are announced on the Internet as they are

discovered, and you can learn about them by going to the cometssection at www.livesky.com The characteristics of their orbits are

published in the same announcements, and if you were to download

the “orbital elements” and follow the directions in Starry Night ’suser’s guide on “adding a comet,” you can add the new comet to

Starry Night’s database and follow its motion across the sky.


Section 5

Deep Space

ONCE we get out of the solar system, the motion

of objects in the heavens becomes a lot less

confusing. Stars, galaxies and other deep space objects

are so far away from us that their position along the

celestial sphere will change little over our lifetime.

This sense of permanence allows us to associate

patterns of stars as constellations, and describe where

a deep space object is with regards to these

constellations. Chapter 5.1 describes the various deep

space objects, chapter 5.2 gives more background on

the constellations, and chapter 5.3 concludes with a

tour of the highlights of the constellations, covering

many fascinating deep space objects along the way.

5.1

STARS AND GALAXIES

Stars

We learned in chapter 1.1 that we classify a star’s brightness

by a number called its magnitude. A star’s magnitude tells you howbright that star appears to your eye as seen from earth, but it tells

you nothing about its intrinsic brightness (the amount of light it

emits). For that reason, magnitude is more properly called apparentmagnitude. To know a star’s intrinsic brightness, you must know

its distance from earth.

Obviously, the closer a star is, the brighter it appears, and manyof the brightest stars in our sky are among the closest. Sirius, Pro-

cyon, Altair, and Alpha Centauri are excellent examples of very

close and very bright stars. But if you could line all the stars up sideby side, at the same distance from earth, you would see that some

outshine others by many thousands of times. The absolute

Section 5 – Deep Space158

Sun

Earth in JulyNearby star

Earth in January

Distant stars

magnitude compares how bright different stars would appear if they

were all the same distance from Earth. As with apparent magnitude,a lower number means a brighter star, and a difference in absolute

magnitude of 1 corresponds to a brightness difference of 2-1/2 times.

The sun has an absolute magnitude of 4.8, which is about averagefor stars. Faint dwarf stars have absolute magnitudes as high as 15,

Determining the Distance to the StarsWe find the distance to the nearest stars by

measuring their parallaxparallaxparallaxparallaxparallax. Parallax is the shift of a nearbyobject against more distant objects when seen from twopositions. A simple demonstration of parallax is to viewan outstretched finger on your hand, first with your lefteye and then with your right eye; notice how your finger“shifts position” relative to the background as yourviewpoint shifts from eye to eye. If you move your fingerfarther away and perform the same experiment, you willnotice that the position shift is smaller. The same ideaapplies to measuring astronomical parallax.

Astronomers record a star’s position, and then record

159Stars and Galaxies

while bright giant stars like Antares and Deneb have absolute

magnitudes as low as -5. To find a star’s absolute magnitude inStarry Night, double-click on the star to bring up its Info Window.

The stars we can see from earth are not representative of all

stars. We see the true powerhouses when we gaze into the nightsky, as almost all the stars visible to the naked eye have a lower

it again six months later when the earth has movedhalfway around the sun and is 300,000,000 km fromwhere it was at the time of the first observation. Thiswill cause the star’s position in the sky to shift, and theamount of this shift is the parallax. More distant starswill shift less, therefore having a smaller parallax. Thetechnique works for relatively nearby stars, but distancesto distant stars, star clusters and galaxies are deter-mined through indirect means because these objectsare so far away that their parallaxes are too small tomeasure accurately. This means that distances to theseobjects must be used with caution. You will see wildlydifferent distances quoted for a star cluster, for example,each of which represents the best effort of anastronomical research project. The latest and presumablymost correct values are used in this book, but they tooare subject to revision and should not be taken literally.

Parallax is usually expressed in arcseconds, and thisexplains the origin of a strange-sounding word. A parsecparsecparsecparsecparsecis the distance an object would have to be if it had aparallax of one arcsecond, and one parsec equals 3.26163light years. The parsec is the most commonly used unitof distance for astronomers.


absolute magnitude (are intrinsically brighter) than our sun. The

most luminous stars are so energetic that they shine like lighthousesacross the gulf of space and can be seen from enormous distances,

while a typical star shines brightly only in its own neighborhood

and the faintest are hard to see even from nearby. The commontypes of stars exist in abundance (in fact, our sun is intrisically

brighter than most stars), but for the most part are invisible without

a telescope.Look closely, and notice that some stars have color. Most stars

appear white, but a few have a slight blue or orange hue. Star colors

are not saturated, and your eye is not very sensitive to colors inlow-light level objects, so star colors are subtle. A reflecting telescope

will enhance star colors, although inexpensive refracting telescopes

often cause stars to exhibit false colors through incorrectly refractedstarlight. Deneb, Rigel, and Spica are bluish, while Antares,

Aldebaran, and Betelgeuse are slightly orange in color. The color

of a star tells you its temperature, and its temperature tells you whatkind of star it is. Red stars are cool and blue stars are hot. Bright red

stars (which actually look orange or even yellow) are “red giants”

or “supergiants”. These giants are hugely swollen stars with bloatedatmospheres. They can be so large that, were one placed at the center

of our solar system, Mars would orbit beneath its surface! Blue

stars shine brightly not because of their size, but because of theirintense heat. Their high surface temperature of 25,000° C for a

blue giant vs. 3,500° C for a red giant (45,000° F vs. 6,300° F)

gives them luminosities of 10,000 times the sun or greater. Otherred stars, called “red dwarfs,” are the most common type of star,

but all are so intrinsically faint that none, including the closest, can

be seen without a telescope. By playing with the sliders in the“Brightness and Contrast” option in the “Sky” menu (the “Settings”

menu in Starry Night Pro), you can have Starry Night draw the

stars in exaggerated shades of blue and red.


Slightly more than half of the stars you see sit alone in space

(disregarding any planets they may have). The rest are accompaniedby a companion star or stars, and these are called double stars, “triple

stars,” or even “multiple stars.” They are very popular targets for

amateur astronomers with telescopes of all sizes, as the telescopeoften reveals two or more stars where the unaided eye sees only one.

Most double stars are true binary systems, and the two stars

are in orbit around a common center of gravity. The star info windowin Starry Night will tell you if a star is a double, and will give the

angular separation between the two components. Open the file

“Mizar” to see a famous multiple star. You begin by looking at thestar Mizar (in the handle of the Big Dipper) with a regular 100°field of view. Use the Zoom button to zoom in. Once your field of

view is down to about 20°, you will see that Mizar is actually twostars, Mizar and Alcor. If you let the cursor hover over each star,

you will note that their distances are similar, indicating that they

are most likely gravitationally bound. Keep zooming in. When youreach a field of view of about 20', you will see Mizar again split

into two stars, officially called Mizar A and B. These stars also

have similar distances and are part of the same star system. Recentobservations have revealed that all three of these stars have faint

companions, so there are actually six stars in this system!

Occasionally two stars happen to line up as seen from earth,even though one lies far beyond the other, and such pairs are called

“optical doubles.” Open the file “AlphaCap” and zoom in to see

this optical double in Capricorn split into two distinct stars. Thebrighter one, Algedi is 109 light years away, while the fainter star is

almost 700 light years distant! Clearly this is just a chance alignment,

not a true multiple star system.Some amateur astronomers enjoy finding and monitoring

variable stars – stars that change their brightness. Some variable

stars, like Delta Cephei, are regular and predictable, pulsating like


clockwork in a cycle that takes a few days. Others, like Mira, follow

a rough pattern that is similar year-to-year but that can hold surprises.Some variable stars have huge changes in their brightness from

maximum to minimum. Mira, for example, goes from a magnitude

of about 3.5 at maximum to 9.5 at minimum, which means a changein brightness of a factor of 200! The star info window in Starry

Night will tell you if a star is variable. One of the areas where ama-

teurs can make a serious research contribution is by monitoringvariable stars, systematically recording their brightnesses, and

forwarding the observations to a centralized authority like the

American Association of Variable Star Observers (AAVSO). There

Naming A StarYou cannot name a star for a friend or loved one.

Although several companies will offer to name a starand to “register” it in an “official document,” for a fee,such vanity registries have no legality. They are simplylistings in the company’s own book and no one else willrecognize the star’s name.

The responsibility for naming stars, as well as comets,newly-discovered asteroids and moons, surface featureson other planets, rests with a committee of the Inter-national Astronomical Union. Although they have beenbusy naming newly revealed craters on Venus, forexample, they do not sell star names.

Star names sold by private companies have no officialstatus, and these companies actually sell nothing morethan pretty pieces of paper at exorbitant prices. Saveyourself the expense and print your own equally-validcertificates on your home computer.


are so many variable stars that it is impossible for professionals to

make all these observations, so they rely heavily on the AAVSO data.

Star Names

The familiar star names that sound so foreign to our ears are,

for the most part, Arabic translations of Latin descriptions. Arabic

astronomers translated into their language the Greek descriptionsof Ptolemy, and centuries later scribes in the Middle Ages copied

and recopied the text, introducing errors while copying words they

did not know, until the origins and meanings of some words becamedifficult to decipher. A few stars names are relatively modern and

some were invented as recently as this century.

This representative list of common and unusual star names givesa feeling for how stars got their names.

Aldebaran: from Arabic for “the follower,” as it follows the Pleia-des Star Cluster across the sky.

Algol: from Arabic “al-gul” – the ghoul; the Arabs apparentlynoticed – and were bothered by – the way it varies itsbrightness.

Antares: from Greek “anti-Aries,” meaning against Mars (ormore colloquially “rival of Mars”). “Aries” is Greek;“Mars” is Latin.

Betelgeuse: a corruption (copying error) of “bad” from “yad al-jauza” – the hand of al-jauza, the Arab’s “Central One.”

Regulus: diminutive of king, (as in “regal”); named by Copernicus

Sirius: from Greek “serios” for searing or scorching

Thuban: corrupted Arabic for “serpent’s head”

Vega: “falling” in Arabic, as the Arabs thought of it as a birdfalling from the sky


Fainter naked-eye stars were given numbers or Greek letters

according to schemes devised centuries ago by John Flamsteed andJohann Bayer respectively when the first detailed star charts were

printed in book form. Starry Night displays common names as well

as Bayer letters and/or Flamsteed numbers in the star Info Window.If a star is too dim to have a common name, a Bayer letter, or a

Flamsteed number, it is identified in Starry Night by its Hipparcos

(HIP) catalog number or its Tycho (TYC) catalog number. Thesetwo catalogs were the product of the Hipparcos project, a mission

by the European Space Agency to calculate the distances to nearby

stars by measuring their parallax (the star distances in Starry Nightcome from this project). The Hipparcos catalog has about 100 000

stars, and the Tycho catalog has about a million. Many other star

catalogs exist, and other texts may refer to a star by its number fromone of these catalogs.

Star Clusters and Nebulae

Anyone who has attended a public “star party,” where amateur

astronomers conduct guided tours of the wonders of the sky withtheir telescopes, has seen at least a few star clusters and nebulae.

They are indeed the showpieces of the night sky. Searching them

out and examining them is one of the sublime and endless activitiesthat delights amateur astronomers (and causes them to crave ever-

larger telescopes).

Star clusters come in two varieties: open clusters (sometimesalso called galactic clusters) and globular clusters. The two have

distinct appearances and histories. Clusters were born together out

of enormous clouds of gas, and the stars of a cluster are all the sameage.

The brightest open star clusters can be seen with the unaided


eye. These include the Pleiades and Hyades clusters in Taurus and

the constellation Coma Berenices. Several others – the Beehive inCancer and the Perseus Double Cluster in Perseus – are visible to

the unaided eye but are best in binoculars or low-power spotting

scopes. (See the descriptions of these objects in the sections on theirrespective constellations in chapter 5.3). Hundreds more await

amateurs with their telescopes. You can search for the most popular

ones by name and display them in Starry Night.Open star clusters take their name from their “open” or unde-

fined shape. They lie in and near the arms of our Milky Way Gal-

axy. Some are forming still, many are quite young (astronomicallyspeaking), and the oldest were born as much as a billion years ago.

The stars in an open cluster are weakly held to each other by gravi-

ty, and the cluster eventually “evaporates” by losing its outermoststars which escape into inter-

stellar space. More than 1,000

are known, and they containfrom a dozen to a few

thousand stars. Each has its

own characteristics, somehave unusual appearances

(like the Pleiades, which re-

sembles a very little dipper),and no two look entirely alike.

Globular star clusters, in contrast, were born during the early

days of the Milky Way and are as old as our galaxy itself. They arehuge compact spherical balls of ten thousand to a million old stars,

and at first glance they all look identical. A closer examination shows

that they have varying degrees of compactness, but differencesbetween them are indeed subtle. About 100 are known in our galaxy.

They form a halo distributed around the center of the Milky Way,

and they avoid the spiral arms. Because they pass near the center of

The Pleiades, an open star clusterThe Pleiades, an open star clusterThe Pleiades, an open star clusterThe Pleiades, an open star clusterThe Pleiades, an open star clusterin the constellation Taurus.in the constellation Taurus.in the constellation Taurus.in the constellation Taurus.in the constellation Taurus.


the Milky Way at some point in their orbit, they appear concentrated

towards that direction, which is in Sagittarius in the summer sky.None are very close to our solar system. Whereas many open star

clusters lie a few hundred to a thousand light years from earth, a

typical globular cluster is tens of thousands of light years distant,and its stars are proportionately fainter. Only a small handful (such

as M13 in Hercules and M22 in Sagittarius in the Northern

Hemisphere and Omega Centauri in the Southern) are visible to theunaided eye, and they look like faint stars. Only through a telescope

can their structure be seen. Open the file “M13” and then zoom in

to see the structure of the Hercules Cluster.A nebula is an enormous cloud of gas (“nebula” is Latin for

cloud). Our Milky Way is permeated with gas, most of it hydrogen

and helium, which is concentrated in its spiral arms. For reasonsnot completely understood, this gas can become concentrated into

comparatively dense nebulae. If they are illuminated by nearby stars,

nebulae shine brightly and are beautiful tenuous wispy clouds whenseen through a telescope; if unilluminated, they appear as blotches

of darkness silhouetted against the myriad stars of the Milky Way.

The brightest, the Orion Nebula, is barely visible to the unaided eyeas a “fuzzy star” in Orion’s sword.

It is impressive through any

telescope if the sky is dark (beinglow-contrast objects, nebulae re-

quire a dark sky to be seen well).

Several others that are pretty whenobserved with amateur equipment

lie in Sagittarius. The most famous

dark nebula is the “Coal Sack” inthe Southern Cross. Nebulae are

the birth-places of stars, and

several are giving birth to brand- M16, the Eagle NebulaM16, the Eagle NebulaM16, the Eagle NebulaM16, the Eagle NebulaM16, the Eagle Nebula


new clusters of stars right now. Hot newborn stars then disperse the

remaining nebulosity. When the Milky Way is much older than now,its gas will be exhausted and no new nebulae – and no new stars –

will form.

Another type of nebula, called a planetary nebula, is entirelydifferent. A planetary nebula is a shell of gas expelled by a aged or

dying star. The Dumbbell Nebula in Vulpecula and the Ring Nebula

in Lyra are pretty sights in amateur telescopes and are popularshowpieces at summer star parties, but most are faint and a challenge

to find. Their confusing name comes from their superficial

resemblance to the planets Uranus and Neptune as seen through asmall telescope. Open the file “Orion Nebula” to see a true nebula,

and then open “Dumbbell” to see a planetary nebula.

Messier ObjectsYou have probably noticed that many of the objects

we have been looking at have the letter ‘M’ followed bya number. These are objects in the MessierMessierMessierMessierMessier catalog. Thereare 110 objects in this catalog, which was compiled byastronomer Charles Messier in the 18th century. Theseobjects are either star clusters, nebulae, or galaxies.Messier was a comet hunter, and his goal in making thiscatalog was to identify “fuzzy” objects which werecommonly mistaken for comets! Today the catalog ismore frequently used as a guide to some of the mostbeautiful objects in the sky. Because Messier was basedin France, the catalog has a definite Northern Hemispherebias, and many treasures in the Southern sky are missingfrom his catalog. Starry Night includes full-color imagesof all the Messier objects.


The Milky Way

Everything we have looked at so far in this book is inside our

own galaxy, the Milky Way. Much as the two-dimensional plane ofthe ecliptic appears as a one-dimensional

line when viewed from earth, the three-

dimensional Milky Way appears as a two-dimensional band of countless stars that

wraps around the sky. This band appears

brighter than the surrounding area, and itsbrightness comes from the combined light

of billions of dim stars. The Milky Way is

the largest and grandest structure in the sky.Open the file “Milky Way” to see the

outline of the Milky Way across the southern

part of the celestial sphere. (If you want tochange the color of this outline to make it

more prominent, choose “Sky | Sky

Settings” (“Settings | Milky Way” in StarryNight Pro), then click on the color bar beside the words “Milky

Way Color” to select another color.) Now choose “Sky | Milky Way”

to turn off this outline. You can still see that the area of the skyinside the Milky Way has many more stars than the surrounding

regions. Turn the Milky Way outline back on and scroll around to

see what other constellations the Milky Way passes through. Youcan see that the Milky Way is not uniformly wide and bright. It is

much brighter and wider in the portion that extends from Sagittarius

south to the Southern Cross, and it is comparatively thin in theopposite direction, towards Perseus and Auriga. This asymmetry

shows us the direction to its center, which lies about 28,000 light

years from us in the direction of Sagittarius.

A view of the MilkyA view of the MilkyA view of the MilkyA view of the MilkyA view of the MilkyWay from Starry Night.Way from Starry Night.Way from Starry Night.Way from Starry Night.Way from Starry Night.


The Milky Way has a third dimension: depth. Because we are

inside the Milky Way, we cannot view its entire extent. However,by mapping the distribution of stars and gas, scientists know that it

would look like a giant flat pinwheel with a bright center and spiral

arms made of stars and clouds of gas, all of which is surrounded bya halo of dispersed stars and unknown material. This structure is

known as a spiral galaxy. Open the file “M100” to see another spiral

galaxy, one that looks very much like our Milky Way would fromthe outside.

The Milky Way is over 100,000 light years in diameter but

only a thousand light years thick. It contains between a hundredbillion and a trillion stars. The very center of the Milky Way seems

to be inhabited by a giant black hole with the equivalent mass of 2.6

million suns. The black hole is surrounded by spiraling clouds ofhot hydrogen gas. None of this can be seen by our eyes because of

numerous dust clouds which lie between us and the center, but it

has been detected and mapped by radio astronomers.Surrounding the Milky Way’s core is the central bulge which is

several thousand light years in diameter and which contains billions

of stars. You can see part of the central bulge on a summer eveningwhen you look at the Great Sagittarius Star Cloud.

Radiating from the center are a suite of spiral arms. No one

knows what causes the arms to form or what maintains theirstructure. They are bright because they contain gas and the hot young

stars that were recently born out of the gas. The Small Sagittarius

Star Cloud is a small portion of the innermost arm, which is about6,000 light years distant.

The spiral arms rotate around the center. At our distance from

the galaxy’s center, the sun takes 230 million years to make onerevolution – a period of time called a “galactic year.” We are moving

towards Cygnus as we orbit the Milky Way.


Our sun lies on the inside edge of the Orion Arm. When we

face Orion, we are looking at the nearest arm to us, which is part ofthe reason there are so many bright stars in Orion’s vicinity. The

Orion Nebula is near the arm’s center. Beyond lies the Perseus Arm

at a distance of about 7,000 light years; the Perseus Double Clusteris within it. Beyond lies the sparse outer edge of the Milky Way.

All the stars that make up the constellations inhabit a tiny portion

of the Milky Way. We see just our neighborhood. The naked-eyestars are less than 2,000 light years from earth (most are much closer),

and this is less than 2% of the diameter of the Milky Way.

Binoculars (or a low-power wide-angle spotting scope) are thebest tool for exploring the Milky Way. Take the time to explore it at

leisure when you are away from city lights.

As much as we’ve learned about the Milky Way, seriousquestions remain unanswered. We do not know how it formed, how

it acquired its globular clusters, the exact nature or history of the

giant black hole at its heart, its precise shape (it seems to have a barstructure running through its center – the “Milky Way Bar”), how

many spiral arms it has, or the nature of the “missing mass” that is

important in holding it all together. Much remains to be discoveredbefore we can claim to know it well.

Other Galaxies

The Milky Way is our galaxy, but billions of other galaxies

exist. You can see three of them with your eyes alone and hundredswith a decent amateur telescope

Early this century galaxies were called “island universes,” which

evokes their size and isolation. Galaxies come in a variety of shapesand sizes, but they have in common that they are vast systems of


many billions of stars, each with its own history. Galaxies are the

fundamental building blocks of the universe, and they stretch awayas far as modern telescopes can see – to distances exceeding 10

billion light years. Billions of galaxies are known.

The closest large galaxy to the Milky Way is the “great galaxyin Andromeda” or more concisely the Andromeda Galaxy. It is also

the brightest galaxy visible from the Northern Hemisphere. It can

barely be seen with the unaided eye on a dark autumn night, but itshows up well even in a small telescope. It lies about 3 million light

years from earth and is comparable to the Milky Way in size. (See

the section on Andromeda in chapter 5.3).Much closer but visible only from the Southern Hemisphere

are the two Magellanic Clouds. These comparatively small galaxies

are satellites of the Milky Way, and they orbit around it like galacticmoons. They will probably collide with the Milky Way in the distant

future and become absorbed into it, as has certainly happened to

other small galaxies that approached too close to the Milky Way inthe distant past. To the eye, they look like detached portions of the

Milky Way. They lie a bit less than 200,000 light years from earth.

Beyond are countless galaxies visible only with a telescope.They look like faint smudges of light, but each has its own character

and its own orientation. The largest have either a spiral shape like

our Milky Way, some with a bar running through their center, orthey are elliptical super-huge balls of stars with little if any gas. A

few are irregular in shape. The most common are small elliptical

galaxies that resemble globular star clusters. It can be hard todistinguish a small elliptical galaxy from a large globular cluster.

Open the file “Elliptical” to see M49, a good example of an elliptical

galaxy. The greatest concentration of nearby galaxies lies in the

direction of Virgo. Our Milky Way is actually on the outskirts of


this huge cluster of galaxies whose center is 60 million light years

from home. Two dozen Virgo galaxies are bright enough to see in

large binoculars.Many amateurs enjoy hunting down galaxies, trying to tease

out their structures, and comparing them to each other.

Two Messier objects which are close together in theTwo Messier objects which are close together in theTwo Messier objects which are close together in theTwo Messier objects which are close together in theTwo Messier objects which are close together in theVirgo Cluster. M87 (left) is an elliptical galaxy, whileVirgo Cluster. M87 (left) is an elliptical galaxy, whileVirgo Cluster. M87 (left) is an elliptical galaxy, whileVirgo Cluster. M87 (left) is an elliptical galaxy, whileVirgo Cluster. M87 (left) is an elliptical galaxy, whileM91 is a spiral.M91 is a spiral.M91 is a spiral.M91 is a spiral.M91 is a spiral.

5.2

ABOUT THE

CONSTELLATIONS

Introduction

In this chapter we return to learn more about the constellations,

the fundamental divisions of the sky.Few constellations look like the animal or person they are named

after, and you should not be frustrated if you cannot see a princess,

bear, or winged horse in the sky. Only in a few cases can the starsbe connected to be made to look like a man, a lion, or a swan. Many

constellations were named in honor of heroes, beasts, and objects

of interest to the people who named them, and not because of anyphysical resemblance. We do similar things today: the state of Wash-

ington looks nothing like the gentleman on the U. S. dollar bill, nor

does the bridge in New York City. The exceptions are few, among


them Orion, Scorpius, Gemini, and Taurus.

During the Renaissance, when star maps were designed to bebeautiful as well as useful, they were filled with elaborate and

colorful drawings that often ignored the background stars. Starry

Night will show constellation figures in the classical style if youchoose, but notice how poorly they fit the stars. Choose

“Constellations | AutoIdentify” and scroll around to see these

classical drawings for each constellation.We learned in the exercise for chapter 1.1 that there is more

than one way of drawing the connect-the-dot stick figures which

we make to identify the constellations. We looked at the two figuresets available in Starry Night: a common astronomical drawing set

and the figures invented by H.A. Rey. His popular book The Stars:

A New Way to See Them remains in print five decades after it wasfirst published. It appeals to people who feel the need to find a one-

to-one correspondence between stars and figures regardless of

whether or not a meaningful fit can be made. In any case, there areno “official” or correct ways to connect the stars, and you are free

to invent your own designs if you like.

Stick figures of the same constellation can vary dramatically. TheseStick figures of the same constellation can vary dramatically. TheseStick figures of the same constellation can vary dramatically. TheseStick figures of the same constellation can vary dramatically. TheseStick figures of the same constellation can vary dramatically. Thesedrawings show the constellation Phoenix in both the standarddrawings show the constellation Phoenix in both the standarddrawings show the constellation Phoenix in both the standarddrawings show the constellation Phoenix in both the standarddrawings show the constellation Phoenix in both the standardfigure (left) and that invented by H.A. Rey.figure (left) and that invented by H.A. Rey.figure (left) and that invented by H.A. Rey.figure (left) and that invented by H.A. Rey.figure (left) and that invented by H.A. Rey.

175About the Constellations

AsterismsSome common star patterns are not actual

constellations. These asterismsasterismsasterismsasterismsasterisms (from aster, the Greekword for star) can be part of one constellation or partsof two or more constellations. These are a few of themajor asterisms; select “Constellations | Asterisms” inStarry Night (“Guides | Constellations | Asterisms” in StarryNight Pro) to display them all.

Big Dipper:Big Dipper:Big Dipper:Big Dipper:Big Dipper: The seven brightest stars of Ursa Major.

Great Square of Pegasus:Great Square of Pegasus:Great Square of Pegasus:Great Square of Pegasus:Great Square of Pegasus: The three brightest stars ofPegasus and the westernmost bright star of Andromeda.

KeystoneKeystoneKeystoneKeystoneKeystone: Four medium-bright stars in Hercules that forma square

Little Dipper:Little Dipper:Little Dipper:Little Dipper:Little Dipper: The seven brightest stars in Ursa Minor.

Sickle:Sickle:Sickle:Sickle:Sickle: Stars in Leo in the shape of a harvesting sickle(or a backwards question mark).

Summer Triangle: Summer Triangle: Summer Triangle: Summer Triangle: Summer Triangle: The stars Vega in Lyra, Deneb in Cygnus,and Altair in Aquila.

Constellations have both a formal Latin name (Aquarius, for

example) and a common English equivalent (Water Carrier). Thisdiffers from star names, which are mostly Arabic, as you learned in

chapter 5.1. Some constellations pre-date the Latin-speaking

Romans, but Latin was the language of scholarship until recentlyand the Latin names were standardized long ago. New constella-

tions invented in modern times (such as Leo Minor) were given

Latin names to conform with ancient custom. Choose“Constellations | Constellation Settings | Labels” (“Guides |

Constellations | Constellation Settings | Labels” in Starry Night Pro)


to choose which style of name (or both) you want displayed in Starry

Night.See Appendix A for a list of constellations and their abbrevia-

tions.

At any given moment you can see half the sky – and approxi-mately half the constellations. As the hours pass and the sky rotates

overhead, constellations in the west set and are replaced by others

that rise in the east. During the course of a long winter night youcan see perhaps two-thirds of the sky, and during the course of a

year even more. But – unless you live on the equator – some con-

stellations remain permanently hidden from view. The far southernconstellations are a mystery to us in the Northern Hemisphere, and

their names – Dorado, Tucana, Centaurus – sound romantic.

Just as the constellations that lie far to the south remain out ofsight (assuming you live at a mid-northern latitude), the

constellations far to the north remain visible all year. They do not

change with the seasons, although they are more easily visible atone time of year than another. Because they are always available,

they seem to have less value and we take them more for granted.

The Big Dipper is above North America for much of the year andhas little novelty – yet Australians never see it.

The most popular and best-known constellations are the 12 that

make up the classical zodiac. This is true even though half of thezodiacal constellations contain few, if any, bright stars and are not

conspicuous, and several cannot be seen from urban areas. They

are well-known because of their astrological associations, but theirfame does not correspond to their visibility. It is ironic that everyone

has heard of Cancer, although few could find it, while comparatively

few people have heard of Auriga, which is a magnificentconstellation of bright stars that is filled with star clusters. Just as

some people are famous because of where they are (the position

they have in our society) rather than who they are, so it is with the


constellations. A constellation’s popularity tells you nothing about

how interesting it is. The zodiac is treated fully in Chapter 3.3.

History of the Constellations

Humans have the need and the skills to find patterns in nature.

Our brains are programmed to invent patterns and to impose order

on disorder. This talent helped our remote ancestors find their wayaround hunting and scavenging sites, and it helps us find our way

around the sky.

We also have a need to feel connected to the cosmos. Beingcompletely inaccessible, the sky is an endless source of mystery

and wonder. It takes a soulless person indeed to gaze up at the sky

on a dark, starry night and not wonder how we fit into it all. Peoplehave been doing just that since the beginning of time.

The origins of most constellations are lost in the mists of

antiquity, and some of them are prehistoric. We can only unravel asmuch as we can of the origins and history of the oldest constellations

by using the sparse clues available to us. Others were created in

more recent times and their history is documented, and the farsouthern constellations were outlined and named during the Age of

Exploration only several centuries ago. As recently as early last

century astronomers were proposing new ones, but their numberwas fixed at 88 early this century and the days of creating new

constellations are over.

The core of our familiar western constellations probablyoriginated in prehistoric Sumeria. The Sumerians, who lived in the

arid land between the Tigris and Euphrates Rivers in what is now

southern Iraq, developed one of the world’s first great civilizationsand counted the invention of writing among their achievements.

They were a superstitious folk, and they paid great attention to corres-


pondences between events in the sky and events on earth. They sawthe seasons as a cyclic battle between the Lion and Bull. In mid-

winter 6,000 years ago, Leo the Lion stood high overhead while

Taurus the Bull lay “dying” on the western horizon, and the lionwas triumphant. The reappearance of the Bull in the morning sky

marked the return of spring and the death of winter, and the Bull’s

turn to triumph. They charted the stars to try to figure out what was

The Oldest Constellation?The Great Bear (the brightest part of which is the

Big Dipper) is probably the oldest constellation, and itdates to prehistoric times. Bears were worshipped in“cave man” days in Europe, before they became extinct(in Europe) at the end of the last Ice Age. Bears are stillworshipped by nomadic people in Siberia.

People named the celestial Great Bear for itsbehavior. It does not look like a bear, but it does act likea bear. Earth bears hibernate, and so does the celestialbear. It’s low in the north in winter and returns in springin a way that reminded people of bears’ seasonalbehavior.

What is truly interesting is that widely separatedpeople around the world – from Europe to Asia to NorthAmerica – saw these stars as a bear. We can only specu-late, but it’s likely that the concept of the Great Bearoriginated during the Ice Age and was carried from Europeto Siberia – or the other way around – and then to NorthAmerica more than ten thousand years ago. If so, thisassociation is one of the world’s oldest surviving culturalartifacts.


happening to them and to the world around them, and their ideas of

interpreting omens were elaborated upon by the Babylonians, wholived in the same area much later. The Babylonians left the first

constellation lists on clay tablets, and they invented the idea of the

zodiac around the 6th century BC.Little Babylonian and less Sumerian constellation lore remains

today, and that which does is imbedded in the classical Greek stories.

The Greeks borrowed the concept of the zodiac from the Babyloniansand incorporated Babylonian star stories into their own myths. Later,

the Romans borrowed the Greek stories and we’ve borrowed those

of the Romans, so there is a tradition of borrowing and elaboratingthat dates back at least 6,000 years.

We know Greek mythology in detail, but less about how they

divided the sky. There are only scattered references to stars and starpatterns from early Greek times.

The astronomer Ptolemy, who lived in Alexandria around 150

BC, described the 48 “Ptolemaic” or classical constellations, whichremained essentially unmodified for 14 centuries. Perhaps 30 of

these are Babylonian in pedigree and the rest indigenous Greek.

His main source was a long poem based on an earlier and now lostwork from about 350 BC by Eudoxus – the Greek astronomer who

constructed the first recorded celestial globe and who worked out

the idea of celestial coordinates. Ptolemy’s book became known asthe Almagest (“the Greatest”) when translated into Arabic. The Arabs

gave most of the stars their familiar common names, which are

usually Arabic translations of the stars’ positions as described byPtolemy. Rigel, for example, comes from Arabic for “foot,” which

is exactly where the star is within Orion. Following the Dark Ages,

the Almagest was translated into Latin, the universal language ofthe Christian world, and reintroduced into Europe around the year

1,000 after an absence of nearly a millennium. The 48 Ptolemaic

star patterns form the core of the constellations of the northern sky.


Many additional constellations were added during the Age of

Exploration. When European navigators first ventured into southernwaters in the late 1500s and early 1600s, they discovered an unchart-

ed sky in addition to uncharted lands. They divided the southern

sky into groups of stars, naming the new constellations after exoticthings they found in the new world, like Pavo the peacock and Indus

the American Indian. Many of these new constellations achieved

legitimacy and permanence by virtue of being included in Bayer’sgreat Uranometria star atlas of 1603. (Bayer also introduced the

idea of using Greek letters to name the brighter stars in this atlas.)

Seven additional constellations appeared in 1690 in a star atlasby the Polish astronomer Johannes Hevelius. He felt that some areas

of the sky were too empty and that his atlas would look more

attractive if these areas of faint stars only were filled in. His newconstellations include Lacerta the Lizard and Vulpecula the Fox.

Hevelius is remembered today as the last astronomer to reject using

the “newfangled” telescope and to observe by eye instead.The last burst of constellation-naming is courtesy of the French

astronomer Nicolas Louis de Lacaille, who lived in Cape Town,

South Africa, from 1750-1752. He created 14 new constellations

Microscopium(left) and Telescopium, two of the SouthernMicroscopium(left) and Telescopium, two of the SouthernMicroscopium(left) and Telescopium, two of the SouthernMicroscopium(left) and Telescopium, two of the SouthernMicroscopium(left) and Telescopium, two of the Southernconstellations named after technological objects.constellations named after technological objects.constellations named after technological objects.constellations named after technological objects.constellations named after technological objects.


from the southern stars, naming them after practical objects like anair pump, a chisel, a microscope, and a pendulum clock. This may

not seem romantic to us, but apparently these were fascinating

objects in their time, and now they too are immortalized in the sky.They contrast so greatly with the mythological beasts and heroes of

the classical age that Lacaille has been accused of turning the sky

into someone’s attic.Through the 18th century, there was no universally approved

list of constellations and no official constellation boundaries.

Mapmakers were free to add new constellations if they wished andto decide where one constellation ended and another began. This

situation was intolerable to astronomers, who were creating ever-

Defunct ConstellationsNot all constellations are ancient. Although the last

wave of constellation-inventing ended when the southernsky was finally filled in the middle of the 18th century,astronomers felt free to create their own as recently asthe late 1800s. Often constellations were created forpolitical purposes – to flatter a patron, for example –but such contrivances were seldom accepted graciouslyby other astronomers, and most disappeared as quicklyas they appeared. They now are minor footnotes in thehistory of the sky. Examples include Robur Carolinium,or Charles’ Oak, invented by Edmond Halley to honorKing Charles II (who once escaped death by hiding in anoak tree); Frederick’s Glory, a sword that honored Prus-sia’s Frederick the Great; and Telescopium Herschelli orHerschel’s Telescope.


more detailed star charts, and an early task of the new International

Astronomical Union was to settle the constellation boundary ques-

The Official CygnusCygnus may look like a cross or even a swan, but

the actual constellation includes many fainter stars thatlie outside the popular stick figure. Since 1928 aconstellation has been defined in the same way that aparcel of property is described on earth – by specifyingits boundaries as a series of interconnected straight lines.The true definition of Cygnus is everything in the skythat lies within these boundaries (the middle two-thirdsof the description is omitted):

Méridien de 19 h. 15 m. 30 s. de 27° 30' à 30° 0'

Paralléle de 30° 0' de 19 h. 15 m. 30 s. à 19 h. 21 m. 30 s.

Méridien de 19 h. 21 m. 30 s. de 30° 0' à 36° 30' m

Paralléle de 36° 30' de 19 h. 21 m. 30 s. à 19h. 24 m.

Méridien de 19 h. 24 m. de 36° 30' à 43°

…

Paralléle de 28° 0' de 21 h. 44 m. à 21 h. 25 m.

Paralléle suite de 28° 0' de 21 h. 25 m. à 20 h. 55 m.

Méridien de 20 h. 55 m. de 28° 0' à 29° 0'

Paralléle de 29° 0' de 20 h. 55 m. à 19 h. 40 m.

Méridien de 19 h. 40 m. de 29° 0' à 27° 30'

Paralléle de 27° 30' de 19 h. 40 m. à 19 h. 15 m. 30 s.

(Reference: Délimitation Scientifique des Constellations,E. Delporte, Cambridge, 1930.)


tion once and for all. A commission was appointed to draw up a list

of official constellations and to define their boundaries, and the

commission’s results were approved in 1928. The boundaries are aseries of “straight lines” in the sky that read like legal property

boundaries on earth. Starry Night will show the constellation bound-

aries and names. Since 1928, no new constellations have been in-vented.

Constellations of Other Cultures

Our familiar constellations are a product of the history of our

western culture. Today, the 88 constellations we know and love areuniversally recognized. Like our Gregorian calendar, they are

“official” around the world. But it was not always so, and each

culture invented its own way of dividing the sky. Sadly, theindigenous constellations of most cultures have been lost and only

in scattered remote areas are pre-western constellations still

remembered.The ancient Egyptians saw a Crocodile, Hippopotamus, the

front leg of a bull (our Big Dipper), and the god Osiris (our Orion).

Their Isis is our Sirius – an important goddess whose reappearancewas used to predict the annual flood of the Nile. The original

Egyptian constellations were replaced by the familiar Greek

constellations after Egypt was conquered by Alexander the Great inthe third century BC, and in the following centuries their ancient

knowledge was almost completely lost. The only clues that remain

of ancient Egyptian sky lore come from enigmatic tomb paintings.The Chinese divided the ecliptic into 28 lunar mansions,

somewhat analogous to our zodiac (which is solar), and the stars

into almost 300 groupings that are smaller than our constellationsand that we would call asterisms. Some patterns that look so obvi-


ous to us that we have a hard time seeing them any other way, like

the W of Cassiopeia, are subdivided by the Chinese (in the case ofCassiopeia, into three). They have four seasonal “super-constella-

tions,” each made of several asterisms with a similar theme: the

White Tiger of autumn, the Black Tortoise of winter, the Blue Dragonof spring, and the Red Bird of summer. The Chinese paid less atten-

tion to star brightnesses than we do when dividing the sky and they

incorporated fainter stars. We are amazed that Lynx is an officialconstellation, so faint are its stars, but the Chinese regularly in-

cluded such faint stars in their asterisms. Among the Chinese

constellations are the Dogs, the Awakening Serpent, the WaggingTongue, the Tortoise, the Army of Yu-Lin, the Weaver Maid, and

the Cow Herder.

Peruvian Dark ConstellationsThe Inca of pre-conquest Peru recognized what we

may call “dark constellations.” At their southern latitude,the center of the Milky Way passes straight overheadand is spectacular. They recognized patterns of brightstars like we do, but they also named the dark dustclouds of the Milky Way. They thought of the dark cloudsas earth carried to the sky by the celestial river. Amongthe dark clouds were Ya-cana the Llama and her babyOon-yalla-macha, the fox A’-toq, the bird Yutu, and Hanp-á-tu the Toad. The Indians watched for changes in thevisibility of the dark clouds, caused by upper-atmosphericmoisture, that would tell them whether the coming yearwould be wet or dry.


The Changing Constellations

Remember that constellations are areas of the sky, and they

have infinite depth, so the stars in a constellation do not necessarily

have anything to do with each other, although they appear to lie inapproximately the same direction as seen from earth. Two stars that

appear to be very close to each other may actually be separated by

enormous distances, one far beyond the other, while stars on oppositesides of the sky may be relatively close to each other with us in the

middle. You cannot tell just by looking. This third dimension of

depth means that the appearance of the constellations depends onyour vantage point.

Distances within our solar system are so small that the

constellations look exactly the same from Mars, Venus, and Pluto(you can verify this with Starry Night). But if we move far beyond

our solar system, the story changes. If our earth orbited a distant

star rather than our sun, the stars would be distributed in completelydifferent patterns and our familiar constellations would not exist. If

aliens live on other planets in orbit around other stars, they have

their own constellations.To see this for yourself, open the file “LDEarth” to view the

stars in the Little Dipper as seen from Earth. Now open the file

“LDAlpha” to view the Dipper as seen from Alpha Centauri, ournearest neighbour (about 4 light years away). Arrange the windows

so that you can see both drawings of the constellations at the same

time. You can see that the bowl of the Dipper has become distortedas viewed from Alpha Centauri. This distortion will increase as we

move farther from our solar system, and the seven stars in the Little

Dipper will make a pattern which doesn’t resemble a dipper at all.Open “LDArct” to see this. You are now viewing the Little Dipper

from Arcturus, which is about 40 light years away from Earth. Clear-

ly the Arcturians would have a quite different name for this star


grouping than the “Little Dipper”! They would probably not evengroup the same seven stars together. Clearly, our constellations are

valid only for our solar system.

Interstellar ExplorationIn addition to all the other adjustments that come

with moving into a new home, humans of the futurewho colonize planets around other stars will have thetask of dividing their sky into their own constellations.It will be a lot of fun.

One of the nearest stars to Earth which we may visitis Barnard’s Star, about six light years distant. Open thefile “Barnard” to see that the new immigrants would seea bright 0th magnitude star near the belt of Orion whichis not in our night sky - because it is our own sun!

The stick figure of Orion, asThe stick figure of Orion, asThe stick figure of Orion, asThe stick figure of Orion, asThe stick figure of Orion, asseen from our Earth (right) andseen from our Earth (right) andseen from our Earth (right) andseen from our Earth (right) andseen from our Earth (right) andfrom Sirius.from Sirius.from Sirius.from Sirius.from Sirius.


Even when viewed from earth, the constellations are not for-

ever. Each star in the sky is actually in motion. Although these speedsare quite high (most stars at moving at several hundred kilometres

per second!), the vast distance to the stars mean that this motion is

imperceptible to us. Each star’s speed and direction is different fromits neighbors, causing each star to move relative to its neighbors as

seen from earth, over very long periods of time. This very slow

motion of a star across the celestial sphere is called its proper motion(“proper” meaning “belonging to,” rather than “correct”). Each star’s

info window in Starry Night shows that star’s proper motion in terms

of the amount by which a star’s RA and Dec will change each year.(Remember that a star’s RA and Dec will also change due to

precession, but precession changes the positions of all stars equally.)

Our familiar constellations look much the same now as theydid at the end of the last Ice Age, but in the distant future they will

become distorted by their stars’ motions and eventually they will

become unrecognizable. The constellations of 1 million AD willbear no resemblance whatsoever to the star patterns we know and

love today. Eventually people will have to invent new constellations.

It would be interesting to know how long people will retain theclassical constellations as they become increasingly distorted before

revising the scheme and starting over.

Starry Night lets you cover a time period of about 15,000 years,but even in this span of time the stars do not move much. However,

we can see some movement in the nearest stars, which appear to

move faster because they are closer. Open the files “LyraPast” and“Lyra Future” and arrange the windows so you can see Lyra as it

looked in 4713 BC, and as it will look in 9999 AD. The stars which

form Lyra hardly change at all, but we can see that the bright starVega has moved relative to the other stars in Lyra. Vega has shifted

its position more than the other stars because it is much closer to

earth. See if you can spot other differences between the two images!

The Big Dipper 100,000 Years from NowThe Big Dipper really does look like a dipper, but it

won’t always. The middle five stars are traveling togetherthrough space on nearly parallel paths as they orbitaround the center of the Milky Way, and they will retaintheir relative spacing far into the future. The Dipper’stwo end stars, however, are traveling in the oppositedirection. Eventually the Big Dipper will become stretchedinto what people may one day call the Big Lounge Chair.

5.3

HIGHLIGHTS OF THE

CONSTELLATIONS

Starry Night shows all 88 constellations in the sky. Many

constellations – especially the newest ones such as Lynx – areobscure and are of little interest. They have no bright stars and it

can be a challenge simply to identify them, even on a dark night

away from city lights. Some are eternally below your southernhorizon (or northern horizon, if you live in the Southern Hemisphere)

and permanently out of sight. But the major constellations are

signposts in the sky and they contain well-known bright stars, starclusters, and nebulae that are within the reach of binoculars or small

telescopes. A few minor constellations also contain objects of special

interest. See Chapter 5.1 for descriptions of the different kinds ofstar clusters, nebulae, galaxies, double stars, and variable stars.

This chapter describes a sampling of bright stars and well-

known deep-space objects within selected constellations. Theconstellations are organized according to the Northern Hemisphere


season when they are best viewed during the evening. See Appen-

dix A for a list of all 88 constellations.There is a file for each constellation described in this chapter.

To view the files for this chapter in the best manner, when you open

Starry Night you should do the following:In Starry Night Backyard, select “Constellations | Constellation

Settings”. Click the Auto Identify button and make sure that the

“Boundary” and “Stick Figure” options (and no other options) arechecked. In Starry Night Pro, double-click on the Constellation Tool

and make sure that the “Boundary” and “Stick Figure” options (and

no other options) are checkedThis will ensure that the constellation files that you open show

the boundary and stick figure of the relevant constellation.

Winter

OrionOrionOrionOrionOrion (the Hunter) (the Hunter) (the Hunter) (the Hunter) (the Hunter) dominates the winter sky. Threebright stars in a row form his belt, two stars mark his shoulders,

and two stars mark his knees or feet. A sword hangs from his

belt.The two brightest stars in Orion are

Betelgeuse and Rigel. They lie at opposite

ends of Orion – Betelgeuse is in his leftshoulder and Rigel in his right foot – and they

have contrasting colors. Betelgeuse is known

for its orangeness, while Rigel is the bluestbright star in the sky. Rigel is also a pretty

double star.

Betelgeuse is enormous. If our sunwere placed at its center, the planets out to

and including Mars would orbit within it. Such

supergiant stars are unstable, and Betelgeuse varies its size and its

191Winter Constellations

brightness slightly and irregularly as it expels material into space.

It is 10,000 times as luminous as our sun and lies at a distance ofabout 520 light years. Betelgeuse is well on the road to exploding

as a supernova.

Rigel is almost twice as distant at 900 light years, and is alsomore luminous (50,000 suns). It is approximately the same size as

Betelgeuse. It is a blue supergiant star with a surface temperature

twice that of our sun. It has a 7th-magnitude companion star 9 arc-seconds distant (their true separation is about 50 times the distance

from Pluto to the sun).

Mintaka (also known as Delta Orionis) is the westernmost ofthe three bright stars that form Orion’s belt. It is one of the prettiest

double stars in the sky for a small telescope. The main star is mag-

nitude 2.2, and its 5.8-magnitude companion lies 53 arcsecondsdistant. At a distance of 1600 light years their true separation is

about 1/2 light year, or 30,000 times the distance between the earth

and sun.The most famous object in Orion is

the famous Orion Nebula (M42) – a true

showpiece in a telescope of any size. It isthe fuzzy naked-eye “star” that marks the

jewel in Orion’s sword. A telescope

reveals it to be a glowing cloud of hothydrogen gas, illuminated by a group of

hot stars (the Trapezium) at its center that

are causing the nebula to fluoresce. The visible part of the nebula is30 light years across and larger in apparent extent than the full moon.

It lies about 1600 light years from earth. A detached portion to the

south (separated by a dark lane of dust that lies in the foreground) iscalled M43. The Orion Nebula’s outer parts extend over 5° from

M42 and cover much of Orion.

Open the file “Orion” to see Orion with major stars labeledand the Orion Nebula.

The Orion NebulaThe Orion NebulaThe Orion NebulaThe Orion NebulaThe Orion Nebula


Canis MajorCanis MajorCanis MajorCanis MajorCanis Major (the Large Dog) (the Large Dog) (the Large Dog) (the Large Dog) (the Large Dog) follows Orion,

the Hunter, across the winter sky. Sirius, thebrightest star in the sky beyond the sun, is

nicknamed the “Dog Star” because it is the

brightest star in the Large Dog. Sirius is8.6 light years (95 trillion kilometers or 50

trillion miles) from earth. It is a hot blue-white

star twice the diameter and 23 times the luminosityof the sun. Ancient Egyptians noticed that Sirius rose

just before the sun as the Nile began to flood, and they

began their year with its appearance. Its famous companion star,Sirius B, visible only in large telescopes, was the first white dwarf

star known; it is a dead star not much larger than the earth but with

as much material as our sun, and is so dense that a tablespoon of itsmaterial would weigh a ton.

Only 4° south of Sirius lies the bright open star cluster M41.

Its 80 stars, the brightest of which are 7th magnitude, fit within acircle 2/3° in diameter. M41 is 25 light years in diameter and 2,000

light years distant. Because of its large size, it is prettiest in binoculars

or a low-power eyepiece.Open the file “Canis Major” to see Canis Major, Sirius, and

M41.

TaurusTaurusTaurusTaurusTaurus (the Bull) (the Bull) (the Bull) (the Bull) (the Bull) is an ancient constellation that dates

back to when bulls were worshipped in the

Middle East. We see it as a face with a redeye and two menacing horns.

Aldebaran – the eye of the bull – is

an orange giant star about 150 times asluminous as our sun. It is one of three

bright orange stars in the sky, the other

two being Betelgeuse in Orion and Antares in Scor-pius. It appears amid the stars of the Hyades Star Cluster, but with a

193Winter Constellations

distance of 68 light years from earth it is actually well in front of

and only half as distant as the cluster’s stars.The face of the bull is outlined by the remarkable Hyades Star

Cluster – the closest bright star cluster to earth. Recent measure-

ments reveal that its center is 151 light years from earth and it has adiameter of 15 light years. A handful of its stars are visible to the

unaided eye, dozens can be seen with binoculars, but the cluster’s

membership totals 200 known stars. The part we see by eye is 5-1/2° in diameter, which corresponds to 15 light years, but this is only

its core; fainter stars lie up to 12° (40 light years) from its center.

This is an old star cluster with an estimated age of 700 million years,and many of its stars are yellow – in contrast to the youthful Pleia-

des with its blue stars.

The prettiest star cluster in theentire sky is certainly the Pleiades,

also known as the Seven Sisters.

The Pleiades rises an hour before themain part of Taurus, and in

Ptolemy’s time it was considered a

separate constellation. A minormystery is why it should be called the “Seven Sisters” when only

six stars are brighter than the rest, but probably the facts were ignored

to fit the mythology of the sisters, who were seven daughters ofAtlas. Some people can see 10 or 11 stars, binoculars reveal dozens,

and the total count is probably over 500. These are young stars born

only about 100 million years ago, and the cluster is not much olderthan the Rocky Mountains. Blue stars, which burn out relatively

quickly, remain, and they illuminate with bluish light a dust cloud

that is passing by (visible only in long photo exposures). The Pleiadesis 380 light years from earth and about 12 light years in diameter.

Open the file “Taurus” in the “Constellations” folder to see

Taurus and to see Aldebaran, the Hyades, and the Pleiades.

The Pleiades star clusterThe Pleiades star clusterThe Pleiades star clusterThe Pleiades star clusterThe Pleiades star cluster


Spring

GeminiGeminiGeminiGeminiGemini (the Twins) (the Twins) (the Twins) (the Twins) (the Twins) are two bright stars – Castor and Pollux

– and two strings of fainter stars that extend from them and thatcan be made to resemble two brothers standing side-

by-side. Their exploits were legendary in the classical

world.Castor is a sextuple star (two, which are

bluish, are visible through a telescope) that lies

52 light years from earth. Yellowish Pollux lies 18light years nearer to earth than Castor and the two are

physically unrelated.

Near Castor’s foot is the bright 5th-magnitude open star clusterM35. It is as large as the full moon and is easily visible to the un-

aided eye on a dark night, and it is a pretty sight in binoculars. The

cluster, which contains at least 200 stars, lies 2,700 light years fromearth and is 20 light years in diameter.

Open the file “Gemini” in the “Constellations” folder to see

Gemini and to see Castor, Pollux, and M35.

CancerCancerCancerCancerCancer (the Crab) (the Crab) (the Crab) (the Crab) (the Crab) is a small constellation with faint stars

only, and it cannot be seen from within a city. Mostpeople have heard of it only because the

sun, moon, and planets pass through it.

Cancer’s major feature of interestis the Beehive Star Cluster (also called

the Praesepe – Latin for “manger”). At 3rd

magnitude, it can easily be seen withoutbinoculars, and with a diameter of more than 1° it

is too large to fit within the field of view of most

telescope eyepieces. The Beehive contains at least50 stars, is 13 light years in diameter, and is 580

195Spring Constellations

light years from earth. The Beehive was one

of four star clusters known in antiquity (theothers being the Hyades, Pleiades, and Coma

Berenices).

Open “Cancer” to see Cancer and theBeehive Star Cluster. Zoom in to see the

Beehive’s brightest stars.

Coma BerenicesComa BerenicesComa BerenicesComa BerenicesComa Berenices (Berenice’s Hair) (Berenice’s Hair) (Berenice’s Hair) (Berenice’s Hair) (Berenice’s Hair) was named in

honor of Queen Berenice of ancient Egypt. It lies one-third of the

way between the tail of Leo and the end of the handle of theBig Dipper

Coma Berenices is both a constellation and a star cluster.

To the unaided eye it is an indistinct cluster of faint starsalmost five degrees across – almost as large as the bowl of the Big

Dipper. Binoculars or a telescope reveals several dozen stars 290

light years distant.Open the file “Coma Berenices” to see the constellation; zoom

in to see the individual stars which make up the open cluster (it is in

the bottom right corner, just beneath Gamma Comae Berenices,which is the last star in the stick figure). Note how the distances of

these stars are all around 290 or 300 light years - if a star’s distance

is much different from this, it is not part of the cluster.

Ursa MajorUrsa MajorUrsa MajorUrsa MajorUrsa Major (the Great Bear) (the Great Bear) (the Great Bear) (the Great Bear) (the Great Bear) is a huge constellation

(the third largest) that lies far to the north and is best seen in thenorthern hemisphere’s spring and summer.

The most famous part of it are the seven

brightest stars which form the Big Dipper(as it is called in the United States and

Canada), but the rest of the bear extends to

the west and south.

M44, the BeehiveM44, the BeehiveM44, the BeehiveM44, the BeehiveM44, the BeehiveCluster.Cluster.Cluster.Cluster.Cluster.


Big DipperThe Big Dipper is so obvious and well known that

you might think it has always been called that – but notso. It is called a dipper only in the United States and

Canada, and there it hasbeen called a dipper foronly about 200 years. Itsorigin is lost in time, butthe idea of a dipper

might have come with slaves from Africa, where theydrank from hollowed gourds. Before water came fromtaps, people used dippers to scoop water out of buckets.

In England the Big Dipper is called the Plough, andin Germany it is the Wagon.

Mizar is the second star from the end of the handle of the Big

Dipper, and is actually part of a multiple star system, as we saw in

chapter 5.1. People with better-than-average eyesight will see aslightly fainter star, Alcor, 12 arcminutes distant. The two names

come from “horse” and “rider” in old Arabic, and according to legend

they were used as an eye test. Mizar is 78 light years distant andAlcor lies 3 light years beyond.

Mizar itself is a very pretty double star in a small telescope. It

has a 4th-magnitude companion star that lies 14 arcseconds distantand that is an easy target for even the smallest telescopes. Their true

separation is at least 10 times the distance from the sun to Pluto.

Mizar was the first double star discovered, in 1650.Open the file “Ursa Major” in the “Constellations” folder to

see the Big Dipper and Mizar and Alcor.

197Summer Constellations

Summer

CygnusCygnusCygnusCygnusCygnus (the Swan) (the Swan) (the Swan) (the Swan) (the Swan) is a graceful bird with outstretched

wings in classical mythology. His head is the star Albireo,his tail the bright white star Deneb, and his wings reach

symmetrically to the side. Cygnus was renamed the

“Cross” by Schiller in his unsuccessful attempt to Christ-ianize the constellations, and the name stuck – although

it is unofficial. It is often called the “Northern Cross” (equally

unofficially) to distinguish it from the Southern Cross.Cygnus straddles the Milky Way in the summertime. Use

binoculars or a low-power telescope to see the myriad of faint stars

within it. The Milky Way is especially rich in this direction becausewhen we look towards Cygnus, we are looking down the length of

one of the spiral arms. The dark “Great Rift” – a split in the Milky

Way that begins north of the middle of Cygnus and that continuesfar to the south – is the effect of enormous dark clouds of gas and

dust that block the light of millions of faint stars that lie beyond

Two especially noteworthy stars are Deneb and Albireo. Deneb,the brightest star in the constellation and one of the stars of the

Summer Triangle, is a true supergiant. Its blue-white color

distinguishes it from other giants which are orange, such as Antaresand Betelgeuse. Deneb is 50,000 times as luminous as our sun. It

lies at the enormous distance of 1,600 light years – so far away that

the light we see tonight left Deneb at the end of the Roman Empire.Albireo, which marks the Swan’s head, is a pretty double star in a

small telescope with contrasting colors (often described as blue and

gold) separated by a comfortable 34 arcseconds. They are giant stars400 light years from earth and at least 650 billion kilometers (100

times the distance from Pluto to the sun) from each other.

Open the file “Cygnus” to see the constellation outline and stickfigure with bright stars highlighted, and the Milky Way.


HerculesHerculesHerculesHerculesHercules (the Strong Man) (the Strong Man) (the Strong Man) (the Strong Man) (the Strong Man) is the ancestor of Super-

man and other superheroes who have extraordinary strengthand skill. He was formerly called the Kneeler for reasons

long lost. Hercules is a rough box of medium-

bright stars – the Keystone – plus others nearbythat can be made to look like a man only with

great difficulty.

Hercules contains the famous M13 Globular StarCluster, one of the showpieces of summer star parties

since it passes nearly overhead and is easy to find. Look for it 1/3 of

the way from Eta to Zeta Herculis on the western segment of theKeystone. At 6th magnitude it is barely visible to the naked eye

under ideal conditions, and through a

small telescope it looks like a small fuzzypatch of light. A large telescope reveals

that it is made of thousands of stars, the

brightest of which are 12th magnitude.The cluster is about 23,000 light years

distant and about 150 light years in

diameter.Open the file “Hercules” to see Hercules and the globular cluster

M13.

LyraLyraLyraLyraLyra (the Lyre) (the Lyre) (the Lyre) (the Lyre) (the Lyre) is a small constellation whose brightest

star, Vega, is the westernmost of the three stars of the

Southern Triangle. A lyre is an ancient stringedinstrument which is the ancestor of the harp and the

guitar. Lyra is overhead during the summer in North

America.Vega is the fifth brightest star in the sky. It is

also relatively nearby at a distance of 25 light years

(55 trillion kilometers or 35 trillion miles). It is a large white star

M13, the HerculesM13, the HerculesM13, the HerculesM13, the HerculesM13, the HerculesClusterClusterClusterClusterCluster


about 50 times as luminous and 2-1/2 times as massive as our sun.

Precession of the equinoxes will cause it to become the Pole Star in14,000 AD. Vega is consuming its fuel rapidly and will burn out in

less than one billion years.

Near Vega is the famous double-double star Epsilon Lyrae.People with near-perfect eyesight can see that this 5th-magnitude

star is actually two “stars” separated by a wide 209 arcseconds.

Under magnification, each star is revealed to be a pair stars separatedby a close 2-1/2 arcseconds. Each star is approximately 100 times

the luminosity of our sun, and they are about 130 light years from

earth.The famous Ring Nebula (M57) is the

best known “planetary” nebula (see

Chapter 5.1) – a tenuous shell of hot gasexpelled from a dying star. Through a

telescope it looks like a tiny slightly-oval

donut just over 1 arcminute in diameter. Itstrue diameter is 1/2 light years and its

distance is over 1,000 light years.

Open the file “Lyra” to see Lyra, the stars Vega and EpsilonLyrae, and the Ring Nebula.

SagittariusSagittariusSagittariusSagittariusSagittarius (the Archer) (the Archer) (the Archer) (the Archer) (the Archer) is a centaur, according tomythology that dates to Sumerian times. Centaurs, who were half-

man and half-horse, combined the skill of men with

the speed of horses and were not to be messedwith. Today Sagittarius is easily seen as the outline

of a teapot. It lies in the southern sky during

summer.The constellation lies in front of an

exceptionally rich part of the Milky Way, whose center

lies within its boundaries, and it contains a wealth of objects for

The Ring Nebula, M57The Ring Nebula, M57The Ring Nebula, M57The Ring Nebula, M57The Ring Nebula, M57


amateurs with telescopes. Only four are described here.

The Lagoon Nebula (M8) is a 5th-magnitude glowing cloud ofhot gas larger than the full moon and bright enough to see with the

naked eye on a dark summer night. Only one other nebula (the Orion

Nebula, which is visible in the opposite season) is more spectacularthrough a telescope. It fluoresces due to the presence of hot young

stars near it that recently formed out of its gaseous material; a cluster

of new stars nearby is called NGC 6530. The nebula is about 5,000light years distant and about 50 by 100 light years in dimension.

The Omega Nebula (M17) is so named

because its arc-like shape resembles thecapital Greek letter Omega. Also called the

Swan or Horseshoe, this fluorescing diffuse

nebula is bright enough to be visible in bino-culars. It is about 6,000 light years distant

and 40 light years across.

The Trifid Nebula (M20) is smaller thanthe nearby Lagoon, but it is easier to see in a

telescope because it has a higher surface brightness. Its name comes

from a complex dust lane that lies in front of and trisects it. TheTrifid is just 1-1/2° from M8 and the two can be seen together in a

spotting scope or binoculars. The Trifid lies 6,500 light years from

earth and is about 30 light years in diameter.M22 was the first globular cluster discovered, in 1665. It is

visible to the naked eye on a dark night, and through a telescope it

rivals M13 in Hercules. M22 would be more magnificent than M13for observers in the United States and Canada were it higher in the

sky, but its southern declination puts it at a disadvantage. M22 is

one of the closest globulars with a distance of 10,500 light yearsfrom earth, and its brightest stars are 11th magnitude – a full

magnitude brighter than the stars of M13.

The Omega NebulaThe Omega NebulaThe Omega NebulaThe Omega NebulaThe Omega Nebula


M22 is one of several bright globular clusters in Sagittarius

(others are M28, M54, M55, M69, M70, and M75). Early thiscentury the astronomer Harlow Shapley proposed that globular

clusters are concentrated towards Sagittarius because they orbit the

center of the Milky Way, and they all pass through this part of thesky before dispersing to the outer parts of their orbits. He concluded

that the Milky Way’s center must lie in the direction of Sagittarius

and estimated the distance to it, displacing the earth from its apparentposition at the center of our galaxy.

Open the file “Sagittarius” to see the constellation and to see

the globular clusters and nebulae within it.

The Closest GalaxyThe closest galaxy to the Milky Way

is the Sagittarius Dwarf Elliptical Galaxy(SagDEG) which is half as distant as thetwo Magellanic Clouds. It circles the MilkyWay with an orbital period of about onebillion years. The 8th-magnitude globularstar cluster M54, long believed to lie withinthe Milky Way, was found in 1994 to be at the enormousdistance of 80,000 light years. This puts it outside thelimits of our Milky Way but inside the SagDEG. It is themost distant globular star cluster visible in all but thevery largest amateur telescopes and intrinsically the mostluminous. Its brightest stars are 14th magnitude.

M54, a starM54, a starM54, a starM54, a starM54, a starcluster in thecluster in thecluster in thecluster in thecluster in thenearest galaxynearest galaxynearest galaxynearest galaxynearest galaxyto our own.to our own.to our own.to our own.to our own.


ScorpiusScorpiusScorpiusScorpiusScorpius (the Scorpion) (the Scorpion) (the Scorpion) (the Scorpion) (the Scorpion) has been called a scorpion

for at least the last 6,000 years, and its stars do indeed look like ascorpion. Too far to the south to see well from

Canada, it is prominent in the south in the summer

sky from the southern United States. At one time itsclaws included the stars of Libra, to the west.

The Scorpion’s heart is a bright red star (actually

orange) called Antares – the “rival of Mars” in Greek becauseit rivals Mars in color and is often close to Mars in the sky (see the

sidebar below). Antares is a true supergiant star 10,000 times as

luminous as our sun and so large that if it were placed where oursun is, Mars would orbit beneath its surface. It lies about 500 light

years from earth – the same distance as the orange star Betelgeuse

in Orion. Like all giant stars, it varies slightly in brightness in anirregular pattern and will soon – astronomically speaking – burn

out.

Beta Scorpii (also known as Graffias) is an exceptionally prettydouble star. The two stars are magnitudes 2.6 and 4.9 with a

separation of 14 arcseconds, and it closely resembles Mizar in the

Mars and Anti-Mars TogetherAntares lies near the ecliptic at an ecliptic latitude

of -4° 34’, and the sun, moon, and planets pass near it.Mars is in conjunction with its rival Antares on the averageof once every two years. Mars-Antares conjunctions occurin March 2001, January 2003, January 2005, December2006, November 2008, November 2010, and so on. Openthe file “Antares” and step through time to see the August23, 2016 conjunction. If you wish, you can continue torun time forward or backward to see other conjunctions.


Big Dipper. The pair is 600 light years distant.

The brightest globular star cluster in Scorpius is M4. It ismagnitude 5-1/2 and it lies only 1-1/3° west of Antares, so it is very

easy to find. This exceptionally close (for a globular!) star cluster is

7,000 light years distant and 55 light years in diameter. Its brighteststars, like M22’s in Sagittarius, are 11th magnitude.

Two especially bright open star clusters are M6 and M7, which

lie above the stinger of the Scorpion’s tail. At magnitudes 5 and 4respectively, they are easily visible to the unaided eye if Scorpius is

high enough in the sky from your latitude, and both can be seen

together in binoculars. They are 2,400 and 800 light years distantand each contains about 80 stars. Ptolemy called M7 a “nebula,”

and its identity as a star cluster was not known until the invention

of the telescope. Because these clusters have very large angularsizes, observe them with very low magnification.

Open the file “Scorpius” to see Scorpius and to see Antares,

Beta Scorpii, and the star clusters M4, M6, and M7.

Autumn

AndromedaAndromedaAndromedaAndromedaAndromeda (the Princess) (the Princess) (the Princess) (the Princess) (the Princess) is a line of three bright stars

and a string of fainter stars that forms a long narrow V in the autumn

sky. Alpha Andromedae is one of thefour stars that forms the Square of

Pegasus.

Andromeda contains one of theprettiest double stars in the sky – Gamma

Andromedae, also called Almach. The two

stars, which are magnitudes 2.2 and 4.9, arenoted for their contrasting colors – yellow and

bluish respectively. This color contrast is shown

well with Starry Night. These are giants 90 and


15 times the luminosity of our sun. The angular separation between

them is 10 arcseconds (1/6 of 1/60 of one degree) and their distancefrom earth is 350 light years.

To cosmologists, the constellation

Andromeda is almost synonymous with theAndromeda Galaxy – the closest and brightest

large galaxy to us. It is visible to the unaided

eye on a dark night as a small fuzzy patch oflight, but even with a large telescope it remains

a disappointingly shapeless blob. We see only

the inner part of what time-exposure photo-graphs reveal to be a huge spiral galaxy of

hundred of billions of stars that spans several

degrees of the sky. It is similar in size and shapeto the Milky Way; our Milky Way would look

something like the Andromeda Galaxy from a

similar distance. At a distance of almost 3million light years, it is the most distant object

you can see without a telescope. It has two satellite elliptical galaxies,

M32 and NGC 205, which are easily visible in small telescopes.The Andromeda Galaxy is also known as M31 – the 31st object in

Charles Messier’s catalog of comet-like objects.

Open the file “Andromeda” to see the constellation outline andstick figure with its major stars and M31 highlighted.

CepheusCepheusCepheusCepheusCepheus (the King) (the King) (the King) (the King) (the King) is an inconspicuous five-sided figurenext to Cassiopeia in the far northern sky. From the

northern United States it is circumpolar, and it never

sets.Although just a faint star to the unaided eye,

Delta Cephei is one of the most important and most

studied stars of all. It is the prototype of the famous

M31, theM31, theM31, theM31, theM31, theAndromeda GalaxyAndromeda GalaxyAndromeda GalaxyAndromeda GalaxyAndromeda Galaxy

205Autumn Constellations

Cepheid Variable Stars – unstable giant stars that pulsate in size

and that vary in brightness in a regular way. The time it takes aCepheid star to go from its maximum brightness down to its

minimum brightness and back to its maximum again is called its

period. The intrinsic brightness of these stars is related to the lengthsof their periods, and this makes them a powerful tool for finding

distances to galaxies. Because there is a one-to-one relationship

between the period of variability and the brightness of a Cepheidvariable, once its period is measured its intrinsic brightness is

known, and since we can observe how bright the Cepheid appearsto be, we can calculate its actual distance. The largest telescopescan see Cepheid Variables in galaxies 50 million light years distant.

Delta Cephei is approximately 3,000 times more luminous than

our sun and it lies some 1000 light years from earth. It varies inbrightness by a factor of two from magnitude 3.6 to 4.3 in a period

of precisely 5 days, 8 hours, and 48 minutes. Compare it to nearby

4.2-magnitude Epsilon Cephei and 3.4-magnitude Zeta Cephei tomonitor its regular change in brightness from your backyard.

Open the file “Cepheus” to see Cepheus in stick figure outline,

and Delta Cephei.

PerseusPerseusPerseusPerseusPerseus (the Hero) (the Hero) (the Hero) (the Hero) (the Hero) is a scraggly stream of stars near

Cassiopeia and Andromeda in the autumn sky.Algol is the “demon star,” its name coming

from Arabic for “the ghoul.” By the time of the

ancient Greeks, observers were alarmed that the starbrightens and fades in a regular pattern. Today we know

that the star is actually two stars in orbit around a

common center of gravity and so aligned that they eclipseeach other as seen from earth. Normally they appear as a

single star of magnitude 2.1, but when the brighter star is hidden by

the fainter, it fades to magnitude 3.4 for about 10 hours. The entire


cycle lasts 2 days, 20 hours, and 49 minutes. Compare with the

nearby star Gamma Andromedae, which remains at a constantmagnitude 2.1, and Epsilon Persei, which is magnitude 2.9. Algol

is about 100 light years distant. (see the section on variable Stars in

Chapter 5.1)The Perseus Double Star Cluster is one of the more striking

naked-eye deep-space objects in the sky. Visible to the eye as two

“out-of-focus” stars that lie near to each other in the Milky Way,binoculars reveal them to be two giant but distant clusters of

hundreds of stars. Their centers are 1/2° apart, but their edges

overlap. The clusters are each magnitude 4-1/2, and their brighteststars are 6th magnitude. At a distance of about 7,200 light years,

these stars are true giants with 100,000 times the luminosity of our

sun. The clusters lie in the Perseus Arm of the Milky Way – the nextspiral arm beyond the Orion Arm.

Open the file “Perseus” to see Perseus and Algol. The Perseus

double cluster is not included in Starry Night Backyard as a separateobject, but this file is set up so that your view is centred on the

cluster. If you zoom in to a field of view of about 4°, you will see

two bright clusters of stars just below the centre of the screen. Ifyou have Starry Night Pro, you can turn on the outlines of the two

clusters by choosing “Sky | NGC/IC”.

The Southern Sky

As we learned in chapter 3.2, the Equator is the only place onearth where all stars are above the horizon at some time. The farther

north or south that we move from the equator, the more stars are

forever hidden to us. The constellations described in this sectionare located south of the celestial equator and cannot be seen well (if

at all) in the North.

207Southern Sky Constellations

CarinaCarinaCarinaCarinaCarina (the Keel) (the Keel) (the Keel) (the Keel) (the Keel) is part of the classical Ptolemaic con-

stellation Argo, the ship. Lacaille subdivided this enormous ancientconstellation into three, of which Carina is

the southernmost part.

Canopus, the second brightest star inthe sky (after Sirius), is named after the pilot

of the fleet of Spartan ships that sailed to

Troy to recover Queen Helen. It is barelyvisible from the southernmost parts of the United

States. Canopus is a yellowish supergiant star with a diameter 65

times that of our sun, a luminosity of 15,000 suns, and a distance of310 light years from earth.

The giant and explosively variable star Eta Carinae is

surrounded by a nebula where stars are now forming. The EtaCarinae Nebula – the largest in the sky – is visible to the naked eye

as a huge splotch of light 2° across. It is over 9,000 light years from

earth. A dark cloud in front of it is called the Keyhole Nebula.Open the file “Carina” to see Canopus and the star Eta Carinae.

Although an image of the Eta Carinae nebula is not included in

Starry Night, you can select the star Eta Carinae and then choose“Edit | Online Info” (“Selection | Online Info” in Starry Night Pro)

to get information and images of this nebula.

CentaurusCentaurusCentaurusCentaurusCentaurus (the Centaur) (the Centaur) (the Centaur) (the Centaur) (the Centaur) lies well below the celestial

equator. Its northern part can be seen from the southern States, but

its southernmost stars cannot be seen from the United States (otherthan Hawaii). The closest star to earth, other than the sun,

is the triple star system collectively known as Alpha

Centauri. It lies 4.3 light years from earth, or300,000 times as far as our sun. Alpha Centauri

A (also known as Rigil Kentaurus) has a bright

companion (Alpha Centauri B) which orbits in an 80-


year period; they presently are 14 arcseconds apart and a magnificent

pair in a small telescope. Another companion, Proxima Centauri, isa faint red dwarf star trillions of light years distant from the main

two and slightly closer to earth than the main pair. Beta Centauri

(also known as Hadar) lies less than 5° to the west of Alpha, and thetwo are a striking pair of very bright stars near the Southern Cross.

The 4th-magnitude Omega Centauri Globular Star Cluster is

the biggest and most luminous globular star cluster in the MilkyWay (it has perhaps a million stars) and also the brightest in our

sky. Its nature was discovered in 1677 by Edmund Halley (of

Halley’s Comet fame); earlier it was mistakenly considered a star,which is why it has a Bayer letter. To the naked eye it is a fuzzy ball

of light, but through a small telescope it is a stunning sight. It lies

approximately 50,000 light years from earth. There is not an imageof Omega Centauri in Starry Night, but Starry Night Pro owners

can locate its position by choosing “Selection | Find” and typing in

“Omega Centauri”.Open the file “Centaurus” to see Centaurus and Alpha and Beta

Centauri.

Crux Crux Crux Crux Crux (the Southern Cross) (the Southern Cross) (the Southern Cross) (the Southern Cross) (the Southern Cross) is the smallest

constellation. The compact group of five bright stars is the

best-known star pattern in the southern sky. It guides youto where the South Star would be if there were one: follow

the long arm of the cross from Gamma through Alpha and

continue it 4-1/2 cross-lengths (27°) southward to find theSouth Celestial Pole. The Southern Cross is a familiar icon

in Australia and New Zealand, where you will see it on everything

from flags and stamps to beer. It is visible from Hawaii and pointssouth.

The Jewel Box Open Star Cluster (also knows as the Kappa

Crucis Cluster) is a large 4th-magnitude open star cluster that is

209Southern Sky Constellations

easily visible to the naked eye and very pretty in binoculars. The

Jewel Box is 20 light years across and about 7000 light years distant,and it contains at least 200 stars, the brightest of which are 7th

magnitude.

One of the most unusual objects in Crux is what looks like ahole in the Milky Way that is about 5° in diameter. The Coal Sack

Dark Nebula is a huge cloud of dark dust, 20 to 30 light years across

and perhaps 600 light years distant, that blocks the light of starsbeyond. It is the most conspicuous of the many dark clouds that lie

along, and partially obscure, the Milky Way.

Open the file “Crux” to see the Southern Cross, the Coal Sack,and the Jewel Box. To find the Jewel Box, zoom in on the star labeled

“Mimosa”. Then zoom in to a field of view of about 5°. The Jewel

Box is visible as a patch of stars about 1.5° directly above Mimosa.The Coal Sack is the region with very few stars which is up and to

the left of the Jewel Box.

This concludes our brief tour through the constellations, but it

barely scratches the surface. The night sky is filled with uncountablewonders, and with a telescope you could explore the constellations

forever.


CONCLUSION

The sky is a fascinating place – especially if you understand it.

It is so removed from our normal experiences that celestial objectsand their slow movements can seem too abstract to relate to. Some

people are overwhelmed and conclude that understanding the sky

is “beyond them;” others (like you) put in the time and trouble topuzzle it all out. You have a lot of help in the form of this book and

software package.

When I was a youngster, I taught myself the constellationsthrough a few books and a rotating star finder, and I learned how

objects in the sky move by reading about them and by studying

diagrams in books. I had to do a lot of mental conversions to connectwords, static illustrations, and tables of figures with what I saw in

the sky, but in time I figured it out. It was not until desktop astronomy

software came along many years later that I was really able tovisualize, for the first time, what these movements looked like. I

had thought about the retrograde loop of Mars and had followed it


through the months – but now I could see it in minutes. I had thought

about why eclipses did not happen each month – but now I couldsee how the paths of the sun and moon intersect and how that inter-

section point moves from year to year. I had thought about how

precession causes the vernal equinox to shift – but now I could skipthrough centuries and watch the equinox precess. I had thought about

how the planets would appear to orbit as seen from the sun – but

now I could put myself at the sun and watch them move. To onewho learned it the hard way, this was magical.

You, dear reader, are fortunate to have a tool such as Starry

Night to help you understand the sky. Work with it, play with it,fool around with it, and gain wisdom.

The reward is a feeling of being connected with the sky. Goodluck, and may you never stop learning about the sky. May you be at

home under it wherever you go.

- John Mosley

APPENDIX A

THE CONSTELLATIONS

Common Name Latin Name Abb. Possessive

Princess Andromeda And Andromeda

Air Pump Antlia Ant Antliae

Bird of Paradise Apus Aps Apodis

Water Carrier Aquarius Aqr Aquarii

Eagle Aquila Aql Aquilae

Altar Ara Ara Arae

Ram Aries Ari Arietis

Charioteer Auriga Aug Aurigae

Herdsman Boötes Boo Bootis

Chisel Caelum Cae Caeli

Giraffe Camelopardalis Cam Camelopardalis

Appendix214


Crab Cancer Cnc Cancri

Hunting Dogs Canes Venatici CVn CanumVenaticorum

Large Dog Canis Major CMa Canis Majoris

Small Dog Canis Minor CMi Canis Minoris

Sea Goat Capricornus Cap Capricorni

Keel Carina Car Carinae

Queen Cassiopeia Cas Cassiopeiae

Centaur Centaurus Cen Centauri

King Cepheus Cep Cephei

Sea Monster or Whale Cetus Cet Ceti

Chameleon Chamaeleon Cha Chamaeleontis

Compasses Circinus Cir Circini

Dove Columba Col Columbae

Berenice’s Hair Coma Berenices Com ComaeBerenices

Southern Crown Corona Australis CrA CoronaeAustralis

Northern Crown Corona Borealis CrB CoronaeBorealis

Crow Corvus Crv Corvi

Cup Crater Cra Crateris

Southern Cross Crux Cru Crucis

Swan Cygnus Cyg Cygni

Dolphin Delphinus Del Delphini

Swordfish Dorado Dor Doradus

Dragon Draco Dra Draconis

Colt Equuleus Equ Equulei

215The Constellations


River Eridanus Eri Eridani

Furnace Fornax For Fornacis

Twins Gemini Gem Gemini

Crane Grus Gru Gruis

Strong Man Hercules Her Herculis

Clock Horologium Hor Horologii

Water Serpent (f) Hydra Hya Hydrae

Water Serpent (m) Hydrus Hyi Hydri

Indian Indus Ind Indi

Lizard Lacerta Lac Lacertae

Lion Leo Leo Leonis

Small Lion Leo Minor LMi LeonisMinoris

Hare Lepus Lep Leporis

Scales Libra Lib Librae

Wolf Lupus Lup Lupi

Lynx Lynx Lyn Lyncis

Lyre Lyra Lyr Lyrae

Table Mountain Mensa Men Mensae

Microscope Microscopium Mic Microscopii

Unicorn Monoceros Mon Monocerotis

Fly Musca Mus Muscae

Carpenter’s Square Norma Nor Normae

Octant Octans Oct Octantis

Serpent Bearer Ophiuchus Oph Ophiuchi

Hunter Orion Ori Orionis

Peacock Pavo Pav Pavonis

Appendix216


Flying Horse Pegasus Peg Pegasi

Hero Perseus Per Persei

Fire Bird Phoenix Phe Phoenicis

Painter’s Easel Pictor Pic Pictoris

Fishes Pisces Psc Piscium

Southern Fish Piscis Austrinus PsA Piscis Austrini

Stern Puppis Pup Puppis

Mariner’s Compass Pyxis Pyx Pyxidis

Reticle Reticulum Ret Reticuli

Arrow Sagitta Sge Sagittae

Archer Sagittarius Sgr Sagittarii

Scorpion Scorpius Sco Scorpii

Sculptor’s Apparatus Sculptor Scl Sculptoris

Shield Scutum Sct Scuti

Serpent Serpens Ser Serpentis

Sextant Sextans Sex Sextantis

Bull Taurus Tau Tauri

Telescope Telescopium Tel Telescopii

Triangle Triangulum Tri Trianguli

Southern Triangle Triangulum Australe TrA TrianguliAustralis

Toucan Tucana Tuc Tucanae

Great Bear Ursa Major UMa Ursae Majoris

Small Bear Ursa Minor UMi Ursae Minoris

Sails Vela Vel Velorum

Virgin Virgo Vir Virginis

Flying Fish Volans Vol Volantis

Fox Vulpecula Vul Vulpeculae

217The Constellations

The “Common Name” is often the English equivalent of the

Latin name, but it can be a descriptive term too (as in the “Twins”for Gemini). The “Abb.” is the official three-letter abbreviation.

The possessive “of” form is used following a star’s name, as in

Alpha Centauri – “the Alpha star of Centaurus.” Those who studiedLatin might recall that the possessive form is the Latin genitive

case.

Appendix218

APPENDIX B

PROPERTIES OF THE PLANETS

DistanceFromSun(AU*)

(Earth radii)

(Earthmasses)

of Year(Earthyears)

of Day(Earthsolardays)

of Orbitto theEclipticPlane (°)

Planet Mean Radius Mass Length Length Inclination

Mercury 0.39 0.38 0.06 0.24 175.94 7

Venus 0.72 0.95 0.82 0.62 117 3.4

Earth 1 1 1 1 1 0

Mars 1.52 0.61 0.11 1.88 1.03 1.9

Jupiter 5.20 11.21 317.83 11.86 0.41 1.3

Saturn 9.55 9.50 95.16 29.42 0.44 2.5

Uranus 19.22 8.01 14.50 83.75 0.72 0.8

Neptune 30.11 7.77 17.20 163.7 0.67 1.8

Pluto** 39.54 0.36 0.002 248.0 6.39 17.1

* 1 AU is the average earth-sun distance, which is 149 600 000 km (92900 000 miles)

** Pluto is sometimes no longer considered a planet

Glossary220

GLOSSARY

absolute magnitude: the apparent brightness (magnitude) a starwould have if it were 32.6 light years (10 parsecs) from the earth. It

is used to compare the true, intrinsic brightnesses of stars. The sun

has an absolute magnitude of +4.8.

annular eclipse: solar eclipse where the moon passes directly in

front of the sun, but is too far from the earth to completely cover the

sun. A ring of sunlight surrounds the moon at the peak of the eclipse.

Antarctic Circle: the line of latitude on the earth’s surface that is

23-1/2° north of the South Pole. The Antarctic Circle marks the

northernmost points in the Southern Hemishpere that experiencethe midnight sun.

aphelion: the point in an object’s orbit where it is farthest from the

sun (helios = sun). The earth is at aphelion each year on aboutJuly 3.

Glossary222

arcminute: a unit of angular measure equal to one sixtieth of a

degree. As an example, the moon has an apparent diameter of about30 arcminutes.

arcsecond: a unit of angular measure equal to one sixtieth of one

sixtieth of a degree, or one sixtieth of an arcminute. As an example,the apparent diameter of Jupiter is about 45 arcseconds.

Arctic Circle: the line of latitude on the earth’s surface that is 23-1/2 degrees south of the North Pole. The Arctic Circle marks the

southernmost points in the Northern Hemisphere that experience

the midnight sun.

apparent magnitude: a system used to compare the apparent

brightness of celestial objects. The lower an object’s apparent

magnitude, the brighter it is. A change in magnitude of 1corresponds to a change in brightness by a factor of 2.5. Objects

with a magnitude of less than 6 can be seen with the naked eye in

good observing conditions.

asterism: a group of stars that people informally associate with

each other to make a simple pattern, such as the Big Dipper and

Square of Pegasus. The stars in an asterism can come from one ormore official constellations.

asteroid: one of the many thousands of chunks or rock or iron thatorbit the sun, also known as minor planets (an older term). Most

orbit between Mars and Jupiter, where they formed, but some cross

the orbit of the earth. Fragments of asteroids are called meteoritesif they fall to the ground.

astrological sign: one of 12 sections of the zodiac that are 30° long

and that corresponds to the positions of the constellations as theywere about 2600 years ago when the astrological system was

established. Do not confuse an astrological sign with an astronomical

223Glossary

constellation with the same or a similar name, as they only partially

overlap.

autumnal equinox: the moment when the sun crosses the ecliptic

in a southward direction on or about September 22. In the Northern

Hemisphere, it marks the first day of autumn. It is also the sun’sposition in the sky at that moment. In the sky, it is one of two

intersection points of the ecliptic and celestial equator (the other

being the vernal equinox).

binary star: two stars which orbit a common center of gravity.

These stars often look like a single star to the naked eye. If morethan two stars orbit a common center of gravity, it is called a multiple

star.

birth sign: see astrological sign.

celestial equator: the projection of the earth’s equator into space;

also a line in the sky midway between the North and South Celestial

Poles. The celestial equator is the line of zero declination in theequatorial co-ordinate system.

celestial meridian: The line of zero right ascension in the equatorialco-ordinate system.

celestial sphere: The projection of the earth into space. The stars

can be imagined to be drawn on the inside of this sphere.

circumpolar: those stars that are so far north (or so far south, in the

Southern Hemisphere) they do not set at all as seen from a given

latitude.

comet: a body composed of ice and dust in orbit around the sun.

conjunction: the passing of one planet by another planet or by themoon or sun. Two planets are in conjunction when they have the

same ecliptic longitude (or alternately the same right ascension).

Glossary224

constellation: one of the 88 portions of the sky that are officially

recognized by the International Astronomical Union (see AppendixA). A constellation is an arbitrary area of the sky, and it includes

everything within that area’s boundaries, regardless of distance from

the earth.

Daylight Saving Time: the adjustment to the clock time that is put

into effect during the summer to extend daylight one hour later in

the evening. In the United States and Canada, Daylight Saving Timebegins on the first Sunday in April (set clocks ahead one hour) and

ends on the last Sunday in October (set clocks back one hour); other

countries change on different dates. Daylight time is one hour aheadof the equivalent standard time ( 6 p.m. standard time becomes 7

p.m. when daylight time is in effect).

declination: the angular distance of an object north of (positive) or

south of (negative) the celestial equator, expressed in degrees. It is

the celestial equivalent of latitude on the earth’s surface. Thedeclination of the celestial equator is 0°; the declination of the North

Celestial Pole is +90°, and the declination of the South Celestial

Pole is -90°.

double star: two stars that appear near each other in the sky. Their

apparent closeness may be due to chance alignment, with one star

far beyond the other, or they may be in orbit around a commoncenter of gravity, in which case they form a binary star.

eclipse: the passage of one object in front of another (as the moon

passes in front of the sun during an eclipse of the sun), or the passageof one object through the shadow of another (as the moon passes

through the shadow of the earth during an eclipse of the moon).

eclipse season: the 38-day period when the sun is near a node of

the moon’s orbit and one or more solar eclipses may happen.

225Glossary

ecliptic: the sun’s apparent annual path through the fixed stars; also

the orbit of the earth if it could be seen in the sky. The 13 astronomicalconstellations that the ecliptic passes through are the astronomical

constellations of the zodiac (12 in astrology).

ecliptic co-ordinate system: the system of specifying positions inthe sky that uses the ecliptic – the sun’s apparent path around the

sky – as the fundamental reference plane. Ecliptic co-ordinates are

useful when specifying positions in the solar system and especiallypositions relative to the sun.

ecliptic latitude: the angular distance of an object above (positive)or below (negative) the ecliptic, expressed in degrees. The ecliptic

latitude of the sun is always zero.

ecliptic longitude: the angular distance of an object, measured alongthe ecliptic, eastward from the vernal equinox, and expressed in

degrees. The ecliptic longitude of the sun is 0° when the sun is on

the vernal equinox, and it increases by very nearly 1° per day throughthe year.

elliptical galaxy: one of the two major types of galaxies, the other

being a spiral galaxy. They often resemble star clusters when seenfrom the earth.

epoch: particular date for which astronomical positions in a bookor table are accurate. Most books give star positions which are valid

for the J2000 (January 1, 2000) epoch.

equation of time: the difference between true solar time (determinedby the sun’s position in the sky) and mean solar time (the time told

by your watch). The two times can vary by as much as 16 minutes

over the course of a year.

equatorial co-ordinate system: a system of specifying positions

in the sky that uses the celestial equator – the projection of the earth’s

equator into space – as the fundamental reference plane.

Glossary226

field of view: angular width of sky that can be seen with an optical

instrument. Field of view is measured in degrees, arcminutes, andarcseconds.

Foucault pendulum: pendulum which varies the direction of its

swing as the earth rotates. Used to demonstrate that the earth rotates,not the sky.

full moon: the moon when it lies directly opposite the sun. Themoon is full two weeks after new moon. The full moon rises at

sunset and sets at sunrise. The earth is between the full moon and

the sun.

galactic co-ordinate system: the system of specifying positions in

the sky that uses the plane of the Milky Way as the fundamental

reference plane.

globular cluster: a huge spherical cluster of tens of thousands of

stars. The stars of a cluster were born together and travel through

space together. M13 and M22 are familiar examples.

greatest eastern elongation: the greatest angular distance to the

east of the sun reached by Mercury or Venus. When a planet is at itseastern elongation, it sets after the sun and is at its best visibility in

the evening sky.

inferior conjunction: the passage of Mercury or Venus betweenthe earth and the sun. The outer planets cannot pass between the

earth and the sun and cannot come to inferior conjunction.

inferior planet: Mercury or Venus, so-called because their orbitsare inside the earth’s orbit around the sun and “inferior” to the earth

in terms of distance from the sun.

Julian Day: the number of days (and fractions of days) that haveelapsed since noon, Jan.1, 4713 BC (Greenwich Mean Time). It is

used to simplify calculating the time interval between two events.

227Glossary

For example, 9:00 p.m. P.S.T. on January 1, 2000, was Julian day

2,451,545.71.

latitude: the angular distance of an object north or south of the

earth’s equator expressed in degrees. The latitude of the equator is

0°, of Chicago is 42° North, the North Pole is 90° North, and Lima,Peru is 12° South. Latitudes south of the equator are expressed as

South or negative.

light year: the distance light travels in one year. One light year

equals 9,460,536,000,000 kilometers or 5,878,507,000,000 miles.

limiting magnitude: the magnitude of the dimmest object that canbe seen through an optical instrument (including the eye). The

limiting magnitude of an instrument will vary with light conditions.

light pollution: the brightening of the night sky due to artificiallight. Light pollution makes it impossible to view many dim objects

that can only be seen in a dark sky.

locked rotation: condition where a moon has the same period of

rotation as its period of revolution around its parent body. This means

that the moon always shows the same face to its parent planet. Ourmoon is in locked rotation around the earth.

longitude: the angular distance of an object east or west of the Prime

Meridian (the line of zero longitude which runs through GreenwichEngland), expressed in degrees. The longitude of Chicago is 88°West.

luminosity: an expression of the true brightness of a star as comparedto the sun. The sun’s luminosity is 1.0 by definition. Sirius has a

luminosity of 23 and Rigel a luminosity of about 50,000.

lunar eclipse: an eclipse of the moon (lunar = moon), caused whenthe moon moves partially or wholly into the shadow of the earth

and grows dark for up to a few hours. A lunar eclipse can be seen by

everyone on the side of the earth facing the moon.

Glossary228

magnitude: an expression of the brightness of a star (or other ce-

lestial object) as it appears from the earth, according to a systemdevised by Hipparchus. Also known as apparent magnitude, to

distinguish from absolute magnitude.Larger numbers refer to fainter

stars, and the brightest stars and planets have negative magnitudes.One magnitude difference is equal to a brightness difference of 2.5

times.

massing: close alignment of three or more planets (or two or moreplanets and the moon), as seen from earth. This occurs when all the

bodies involved in the massing have similar ecliptic longitudes.

meridian: the line in the sky that extends from the southern point

on the horizon through the zenith (overhead point) to the northern

point on the horizon, bisecting the sky into an eastern and westernhalf. Objects are at their highest when they cross the meridian; the

sun is on the meridian at local noon. Also a line on the surface of

the earth (or another body) that extends from pole to pole.

Messier object: one of the 110 objects in the catalog compiled by

Charles Messier. Most Messier objects are galaxies, star clusters or

nebulae.

meteor: the visible flash of light produced when a meteorite falls

through the atmosphere and bursts into flame because of friction

with air molecules; also called a “shooting star” or “falling star.”

meteorite: the solid particle, either stone or iron, that falls through

the atmosphere to produce a meteor. Most meteorites are fragmentsof asteroids. Science museums display meteorites that survived their

falls.

midnight sun: the sun when visible at midnight, which happensonly in summer north of the Arctic Circle or south of the Antarctic

Circle.

229Glossary

Milky Way: our own galaxy.

minor planet: see asteroid.

month: the period of time it takes the moon to orbit the earth (or a

moon to orbit a planet). A sidereal month (27-1/3 days) is the timeit takes the moon to orbit the earth and return to the same position

relative to the stars; a synodic month (29-1/2 days) is the time it

takes the moon to orbit the earth and return to the same positionrelative to the sun and is the time between new moon and new moon.

nebula: a cloud of gas or dust in space, either between the stars or

expelled by a star; nebula is Latin for cloud. There are many kindsof nebulas.

new moon: the moon when it lies in the same direction as the sun

and the beginning of a cycle of lunar phases. The new moon risesand sets with the sun. The moon is between the earth and the sun at

new moon.

night vision: enhanced ability to see objects in the dark. Night vision

is ruined if the eyes are exposed to bright light.

node: the point(s) in the sky where two orbits or paths cross. Thenodes of the moon’s orbit are the two places where the moon’s orbit

crosses the ecliptic.

North Celestial Pole: the sky’s north pole; the point in the skydirectly above the earth’s north pole.

obliquity of the ecliptic: the amount of the tilt of the earth’s axis

(23.5°) which determines the angle the ecliptic makes with thecelestial equator as they intersect in the sky.

occultation: the disappearance (eclipse) of one object behindanother, as a star or planet behind the moon or a star behind a planet.

Glossary230

open cluster: a diffuse association of a few dozen to a few thou-

sand stars, all of which were born together and which travel throughspace together. The Pleiades in Taurus and Beehive in Cancer are

familiar examples.

opposition: the position of a planet when it is opposite the sun inthe sky. Only objects that orbit outside the earth’s orbit come into

opposition; Mercury and Venus cannot.

parallax: the apparent shift in position of an object when it is viewed

from two different points. The parallax of a star is measured from

opposite ends of the earth’s orbit, and for the nearest stars is lessthan one second of arc (one arcsecond).

parsec: the distance an object would have if its parallax were one

second of arc (see parallax). One parsec equals 3.26163 light yearsor 30,856,780,000,000 kilometers.

penumbra: shadowed area in an eclipse where only part of the

light source is blocked. Observers in the penumbral shadow of asolar eclipse see a partial eclipse.

perihelion: the point in an object’s orbit when it is closest to thesun (helios = sun). The earth is at perihelion each year on about

January 3.

Period: the time an object takes to complete a certain motion andreturn to its original state, e.g. period of revolution

planetary nebula: a luminous cloud of gas expelled by an aging

star that has become unstable. The name comes from a nebula’ssuperficial resemblance to the faint planets Uranus and Neptune as

seen through a small telescope. The Ring Nebula M57 in Lyra is a

familiar example.

precession of the equinoxes: the slow wobbling of the earth’s axis

in a 25,800-year cycle, caused by the gravitational attraction of the

231Glossary

moon on the earth’s equatorial bulge. Precession causes the vernal

equinox (and all other points on the ecliptic) to regress westwardalong the ecliptic, slowly changing the equatorial co-ordinate grid.

Prime Meridian: the line of longitude which passes through

Greenwich, England, and which is the zero line for expressinglongitude on the earth’s surface.

proper motion: the motion of the stars relative to each other, causedby their actual motion in different directions at different speeds

through space.

retrograde motion: the apparent “backwards” or westward motionof a superior planet against the background of the stars caused when

the faster-moving earth on an inside orbit passes that planet.

revolution: the orbiting of one body around another body, as themoon revolves around the earth and the earth revolves around the

sun.

right ascension: in the equatorial co-ordinate system, the angular

distance of an object eastward from the zero point (which is the

vernal equinox), usually expressed in hours and minutes (whichrepresents the earth’s rotation from the vernal equinox to the object).

It is the celestial equivalent of longitude on the earth’s surface.

rotation: the spinning of a body on its own axis. The earth rotatesonce a day. See revolution, which is often confused with rotation.

sidereal day: the time it takes the earth to rotate once relative to the

stars, in 23 hours 56 minutes and 4 seconds, which is 4 minutes lessthan a solar day. (During one sidereal day the sun moves 1° east

along the ecliptic, and the earth has to rotate 4 additional minutes to

complete one rotation relative to the sun in one 24-hour solar day.)

sidereal month: the time it takes the moon to complete one orbit of

the earth and return to the same position among the stars, on an

average of 27.32166 days.

Glossary232

sidereal year: the time it takes the earth to complete one orbit of

the sun relative to the stars, in 365 days 6 hours 9 minutes and 10seconds. It is also the time it takes the sun to appear to travel once

around the sky relative to the stars.

solar day: the time it takes the earth to spin once relative to the sun,in exactly 24 hours (by definition).

solar eclipse: an eclipse of the sun by the moon, when the moonpasses in front of the sun. Solar eclipses can be partial, total, or

annular. Only the few people in the narrow “path of totality” see a

solar eclipse as total.

South Celestial Pole: the point in the sky directly above the earth’s

south pole.

spiral galaxy: The second major type of galaxy, characterized by acentral bulge and a number of spiral arms extending from the bulge.

standard time zone: one of the 24 sections of the earth, each about15 degrees wide and extending from pole to pole, within which the

time is the same. In practice, natural and political boundaries

determine the edges of time zones.

summer solstice: the moment when the sun reaches its greatest

distance north of the celestial equator, on or about June 21. In the

Northern Hemisphere this marks the first day of summer; in theSouthern Hemisphere it marks the first day of winter.

superior conjunction: the position of a planet when it is on the far

side of the sun (and in conjunction with the sun).

superior planet: the planets Mars through Pluto, so-called because

their orbits are outside the earth’s orbit around the sun and thus

“superior” to the earth in terms of distance from the sun.

synodic month: the time it takes the moon to complete one cycle

233Glossary

of phases, such as from new moon to new moon, on an average of

29.53059 days.

transit: the passage of one celestial body across the face of a second

larger body.

triple conjunction: close alignment of a planet and a star at three

distinct times, caused by the retrograde motion of the planet. The

planet passes the star once in its forward motion, once more in itsretrograde motion, and a third time when it resumes its forward

motion.

tropical year: the length of time it takes the sun to circle the skyrelative to the vernal equinox. The tropical year is identical to our

standard “year”.

Tropic of Cancer: the line of latitude on the earth’s surface that is23-1/2 degrees north of the equator. It marks the northernmost points

in the Northern Hemisphere from which the sun can appear directly

overhead.

Tropic of Capricorn: the line of latitude on the earth’s surface that

is 23-1/2 degrees south of the equator. It marks the southernmostpoints in the Southern Hemisphere from which the sun can appear

directly overhead.

umbra: shadowed area in an eclipse where the light source iscompletely blocked. Observers in the umbral shadow of a solar

eclipse experience a total eclipse.

Universal Time: in simplest terms, the time at the longitude ofGreenwich, England. Universal Time (UT) is widely used in

international publications as a standard time.

variable star: a star whose light changes over time. Stars can varyin brightness for a variety of reasons, from eclipses by companions

to instability of their interiors that causes the stars to swell and shrink.

Glossary234

vernal equinox: the moment when the sun crosses the ecliptic in a

northward direction on or about March 21. In the NorthernHemisphere, it marks the first day of spring. It is also the sun’s

position in the sky at that moment. In the sky, it is one of two

intersection points of the ecliptic and celestial equator (the other isthe autumnal equinox).

waning crescent: the phase of the moon between third quarter and

new moon. Waning means declining or fading.

waning gibbous: the phase of the moon between full moon and

last quarter.

waxing crescent: the phase of the moon between new moon and

first quarter. Waxing means increasing.

waxing gibbous: the phase of the moon between first quarter andfull.

winter solstice: the moment when the sun reaches its greatestdistance south of the celestial equator, on or about December 22. In

the Northern Hemisphere this marks the first day of winter; in the

Southern Hemisphere it marks the first day of summer.

zenith: the point in the sky directly above the observer; the top of

the sky.

zodiac: the band of constellations or signs that the sun passes throughas it moves around the sky. There are 12 signs of the astrological

zodiac but 13 constellations of the astronomical zodiac.

INDEX

AAAAA

Absolute magnitude 158Age of Aquarius 110Albireo 197Alcor 196Aldebaran 192Algol 205Almach 203Alpha Centauri 207Altitude 20

best for observing 20Andromeda 203Andromeda Galaxy 171, 204Angles

between objects in the sky 30measuring 22

Antarctic Circle 94Antares 202Apollo 11 mission 122Arctic Circle 94Aries 19Asterisms 175Asteroids 149Astrology 107Autumnal equinox 89Azimuth 21

BBBBB

Bayer 180Beehive Cluster 194, 195Beta Scorpii 202Betelgeuse 190Big Dipper 19, 188, 196Binary stars 161

Binoculars, tips on purchasing 58Birth signs 109Black hole 169

CCCCC

CalendarGregorian 119Islamic 119

Cancer 194Canis Major 192Canopus 207Capricornus 108Carina 207Cassiopeia 81Castor 194Celestial equator 88Centaurus 207Cepheid variable stars 205Cepheus 204Ceres 149Co-ordinate systems 63. See also

specific co-ordinate systemsCo-ordinates

displaying with Starry Night69

epoch 106J2000 106

Coal Sack Dark Nebula 209Coma Berenices 195Comets 150

adding to Starry Night 153Hale-Bopp 150, 151Halley's 151tails 151

Page numbers in italics indicate illustrations.For definitions see glossary.

Index236

Conjunctions 146Constellations 17, 173. See also

entries for individualconstellations

as seen from other stars185, 186

asterisms 175changes due to proper motion

187defunct 181displaying with Starry Night

27history 177names 175other cultures

Babylonian 179Chinese 183Egyptian 183Peruvian 184Ptolemaic 179Sumerian 177

seasonalautumn 203spring 194summer 99, 206winter 99, 190

stick figures 174Rey's 174

Copernicus, Nicolas 35Crux 208Cygnus 197

DDDDD

Daysidereal 74solar 73

Daylight saving time 77Declination 66Delta Cephei 204Deneb 197

EEEEE

Eagle Nebula 166Earth

circumference 93lines of latitude and longitude

64phases 121, 122precession 101, 103revolution 85rotation 33, 73seasons 87tilt 87viewed from the moon 122

Eclipses 123annular 45eclipse season 125lunar 127nodes 124solar 45

as seen from the moon 48last total 127observing 46viewing with Starry Night47

Ecliptic co-ordinate system67, 68

Ecliptic line 35, 36, 88Elliptical galaxy 171, 172. See

also entries for individualobjects

Epoch 106Epsilon Lyrae 199Equation of time 90Equatorial co-ordinate system

22, 65, 67Equinox

autumnal 89vernal 66, 87

Eta Carinae Nebula 207Exercise, Starry Night

27, 38, 50

237Index

FFFFF

Field of view 22through different instruments

23Foucault pendulum 34

GGGGG

Galactic co-ordinate system 69Galaxies 170. See also entries

for individual objectsclosest to Earth 201elliptical 171, 172spiral 169, 172spiral arms 169

Galilei, Galileo 35, 138Gemini 194Globular clusters 165. See also

entries for individual objectsGreat Red Spot 143

HHHHH

Hale-Bopp 150, 151Halley’s Comet 151Heliocentric model, proof of 35Hercules 198Hercules Cluster 198Hipparchus 101Hipparcos project 164Horizon co-ordinate system. See

Local co-ordinate systemHyades 193

JJJJJ

Jewel Box Open Star Cluster 208Julian day 78Jupiter 143

Great Red Spot 143moons of 143

KKKKK

Kepler, laws of planetary motion132

LLLLL

Lagoon Nebula 200Latitude 65

lines of 92Light pollution 55

displaying in Starry Night 31moonlight 56

Light year 25Limiting magnitude 24

of different instruments 24varying in Starry Night 31

Local co-ordinate system 20displaying with Starry Night

29Local meridian 21Locked rotation 123Longitude 64Lyra 187, 198

MMMMM

Magellanic Clouds 171Magnitude 24

limiting 24of planets 24

Mars 140moons 142observing 142retrograde motion 140

Mercury 135Meridian, local 21Messier objects 167. See also

common names of individualMessier objects

Meteor showers 152Meteors 150

Index238

Milky Way 168Minor planets 149Mintaka 191Mithraism 102Mizar 196Month

sidereal 118synodic 118

Moon 115. See also entries forindividual planets

dark side 121full 117gravitational influence 103locked rotation 123midnight 120motion 43, 119new 117orbital nodes 115, 124parallax 116phases 44surface 121viewing from the surface of

122

NNNNN

Nebulae 166. See also entries forindividual objects

planetary 167Neptune 145Night vision, accessories for

preserving 57North Celestial Pole 104North Star 80

altitude and azimuth 29other pole stars 105

OOOOO

Obliquity of the ecliptic 87Occultations

of planets 130, 146

of stars 130Omega Centauri Globular Star

Cluster 208Omega Nebula 200Orion 19, 23, 186, 190Orion Nebula 191

PPPPP

Parallaxmoon 116stars 158

Penumbraof Earth's shadow 127

Perseus 24, 205Perseus Double Star Cluster 206Planetary nebulae 167. See also

entries for individual objectsPlanets 131. See also entries for

individual objectsconjunctions 146inferior

cycle 133magnitude 24massing 147motion 49occultations 146superior

cycle 139transits 134triple conjunction 148

Pleiades 165, 193Pluto 145Polaris. See North StarPollux 194Precession 86, 101, 103Prime Meridian 64Proper motion 187Ptolemy 179

239Index

RRRRR

Retrograde motion 140Rigel 191Right ascension 65Ring Nebula 199

SSSSS

Sagittarius 199Sagittarius Dwarf Elliptical

Galaxy 201Saturn 144

moons 145rings 144

Scorpius 99, 202Seasons 87Sidereal day 74Sidereal year 85Sirius 192Solar day 73South Celestial Pole 84South star 83Spiral galaxy 169, 172. See also

entries for individual objectsSquare of Pegasus 203Star clusters 164. See also

entries for individual objectsglobular 165open 164

Star of Bethlehem 148Star trails 33, 34, 80Stars 23, 157. See also entries

for individual objectsabsolute magnitude 158Bayer letter 164binary 161catalogs 164Cepheid variable 205circumpolar 38, 80colors 160distances 25

Flamsteed number 164magnitudes 24names 163parallax 158proper motion 187red giants 160seasonal changes 37variable 161

Summer solstice 89Sun

changes in elevation withlatitude 92

midnight 95rise and set points on the

horizon 97seasonal elevation changes 91

TTTTT

Taurus 192Telescope

Dobsonian 60reflecting 60refracting 60Schmidt-Cassegrain 61spotting 60

Thuban 104Time zones 76Trifid Nebula 200Triple conjunction 148Tropic of Cancer 92, 108

shifting position 108Tropic of Capricorn 93Tropical year 86

UUUUU

Umbraof Earth's shadow 127

Universal time 78Uranus 145Ursa Major 195

Index240

VVVVV

Variable stars 161Vega 198Venus 137

phases 138Vernal equinox 66, 86

WWWWW

Winter solstice 89

YYYYY

Yearsidereal 85tropical 86

ZZZZZ

Zenith 20Zodiac 35

13th Constellation 37extended constellations of 111watching Sun move through

with Starry Night 41

Documents

STARRY NIGHT COMPANION